Sibling relationships are complicated. GM–player relationships can be or become complicated. Combining them can be a very complicated snarl. Having played with one brother and his friends (a slightly different but related social dynamic) and having played in games when the GM is a sibling to the other players, I'm familiar with some of the less obvious undercurrents that influence the game. As a grown adult who has since played with many more people, I also have more perspective and understanding of those situations from my past than I did at the time, and a good sense of how unusual the social dynamic is when playing with siblings.
When your brothers sit down to play a roleplaying game with you as their GM, they are submitting to a subordinate position in a power relationship. They don't like this, whether they conscious think of it like that or not. Disrupting your game is how they take back some of that power.
You have three options here, in order of difficulty: suffer gracefully, reset the player–GM relationship, and quit. All of these are going to be framed in terms of taking back control over your own game experience without trying to fruitlessly control your brothers' behaviour. (Have you ever successfully controlled your brothers? Why should it work now?)
Joking around and making stupid noises are intended to disrupt your game, and take you down a few pegs from your high GM chair. You can choose to not let this bother you, and you can do it with some practice. (Yes, it will take practice.)
When they start being disruptive, just stop GMing. Let go control of the table. Say something like, "Hey guys, let me know when you're ready to play." Don't sound angry. (Don't be angry.) Don't snap, or sneer. Treat the disruption like it's perfectly fine, like it's the most normal thing. Then use your sudden free time to do something else at the table.
Re-read your favourite bits from the rulebook. Review your notes. Work on that dungeon that you're planning to use a few sessions from now. Draw. Basically, do any of the things that a bored player normally does to entertain themselves when the game isn't keeping them engaged anymore.
Don't ignore them. If they talk to you, respond. Pretend as if you've all agreed to take a break to relax for a bit. Answer rules questions, respond to jokes. Don't try to passive-aggressively "punish" them for stopping the game.
The key is to not give them the satisfaction of annoying you. You are keeping control of your own happiness. You're not keeping control of the game (you never were in control, actually), and you're decoupling your own peace of mind from the success of the RPG, which means they don't have an easy link to you, in the form of you caring about the game, to indirectly mess with you.
If they never return to the game, then you and they weren't going to get any actual playing done today anyway. Now you know. Eventually, you can stop reading your notes or whatever and leave the table. Go have lunch or play a videogame, or whatever else you have to do.
If they actually do want to play, they'll settle down, get quiet, and start looking at you as if they're waiting. They might actually say, "Okay, we're ready to play." Then you can pick right back up where you left off. And if they start being obnoxious again, repeat the whole thing.
Most importantly, never let them see you annoyed. That's what they're after, so don't give it to them. You'll be happier, and you'll remove their incentive for disrupting the game.
Reset the Player–GM Relationship
This is the result of taking the techniques of Suffer Gracefully to their logical extremes. Simply refuse to GM for them when they're being obnoxious. Right now, your own desire to play is keeping you at the table, so they don't need to behave to keep you there. You need to change that.
Doing this requires all the same advice from above. Don't be annoyed, or at least don't show them that you're annoyed. Pretend as if you've agreed to not play right now. Pretend that when they say "*fart noises* HAHAHA" they actually said, "Hey, I'm not really in the mood for RPing today, let's do something else."
Then just go do something else. If they try to call you back, just tell them that you don't want to GM if they don't want to play. If they say they want to play, answer them with "No, that's OK! I can go do something else," as if they asked you, "Hey, do you want to play?" By speaking as if you are deciding that you just don't want to play, you mess with their heads a little bit. You're speaking as if they are the ones who said they don't want to play, and by doing that you force them to talk to you as if they admitted they don't really want to play right now.
Walking away might feel like giving up, or giving in to them. It might feel like letting them "win." It's the opposite, actually. If you're not willing to walk away, then they can control you by controlling the game. By walking away, you show them that you are not stuck in their trap.
Now, if they say they'll stop being disruptive and to please come GM, that's the beginning of resetting the relationship. Go give it another shot. Use Suffer Gracefully if necessary, and if necessary, walk away again. (Don't do it three times though. Then they know they can be disruptive, but you'll come back every time. Also, it's very hard to not show some annoyance through three times of them changing their minds. Three is a magic number. Don't give them three chances.)
The point of this exercise is to get them to value what you do. If you are willing to walk away, then they actually have to think about how much they want to play in your game, and they have to start acting like they want to play in order to actually have you run the game for them. You're taking back control of your time. They will realise that they have to behave if they want to play, and they'll realise it without you having to lecture them (because lecturing is an attempt to regain control over them that will only give them yet another opportunity to resist your authority and mess with you more).
Again, all the same advice applies. Don't be passive-aggressive. Don't say that they don't deserve to play in your game. Just act like it's totally normal for them to sit down to play, and then "change their mind". Start hearing the disruptions as "hey, we changed our mind and don't want to do this" and take it gracefully. Go do something you enjoy.
Eventually, they'll either start behaving better in order to keep you at the table, or they won't improve their behaviour. If they keep acting like hooligans, then you know that you were the only one who actually wanted to play. Then you can either go back to Suffer Gracefully and just put up with it in order to play at least a bit, or you can move on to the advanced class: quitting.
If your brothers don't really want to settle down and play, you don't actually have a roleplaying group. In this case, the only person who actually wants to play a roleplaying game is you, and your brothers aren't at the table to roleplay, they're just at the table for a chance to mess with you and ruin your fun. You're only sensible option is to not play.
This is actually unlikely, but it's a possibility that you have to accept – just because your brothers are the only people around who say they'll play, doesn't mean that sitting down at a table with them will actually result in playing. Accept that you might not have a group at all, and you'll find it easier to quit – if quitting becomes the only obvious option left to you.
Again, the same advice goes here. Don't give them the satisfaction of ruining your fun. Don't show them that you're annoyed. Just refuse to play. Suggest they run a game for each other if they pester you to GM.
Read your RPG books, browse your RPG forums, read RPG blogs – feed your desire for roleplaying some other way. Brothers will always be brothers, but you won't always live with them. Keep the fire of your hobby burning, but wait until you can find people who actually want to play before you decide to GM again. It might suck to not play, but if you've tried Suffer and Reset and neither have made playing with them tolerable for you, then taking a sabbatical is your only remaining option.
It might come to pass that your brothers regret losing their GM. They might, after a while, convince you to GM. Accept gracefully, but don't commit to an ongoing campaign. Say you'll run a one-shot, and that's it. Say, you'll see how it goes. Don't tell them so, but think to yourself that they're on probation now – if they behave, then consider running for them again. If not, then you haven't invested much time or hope into getting to play again, and you can return to your sabbatical.