In our last session a melee style rogue using Sneak Attack did more damage than the warrior. That seems pretty powerful especially at low levels.
Is there no way creatures can defend against a Sneak attack?
Is there no way a creature can stop the Sneak attack?
If it's able to impose disadvantage on the rogue's attack, then the rogue will not be able to do a sneak attack, even with support. However, imposing disadvantage on somebody is not that easy and most enemies may not have this ability.
Especially in early levels, rogues outdamage warriors, if there is a single target that is attacked by two characters. The rogue will get the sneak attack and that is indeed powerful.
However, that assumes that you have two people to attack a single enemy. Speaking in football terms, when you have your two front line people fighting the same enemy and you have as many enemies as characters, now there is one free to do as he pleases. Blitz the Quarterback, or rather the mage for example. So assuming your mage does not want to get into melee, you now have two enemies free (the one that the mage did not engage and the one that your warrior/rogue combo did not engage when they doubled on one enemy).
So while your rogue is making his powerful sneak attack, a number of things may happen:
Yes, the rogue is powerful. But using his power might be tactically unwise, putting the weaker party members in danger if the enemy is not outnumbered from the start.
If the party is outnumbering the enemy, chances are the enemy itself is pretty strong. So the rogue still has to run the risk of getting smacked because he needs to go close. And he does not have the staying power of a warrior class.
So the answer to the implied question of "how to balance this power": you need smart enemies. Enemies that employ tactics. If they just stand there in line, waiting to be slaughtered, then the rogue is overpowered. Let them be smart, let them exploit the tactical weaknesses of the rogues somewhat egoistic power.
And in the end of the day, the rogue would not be able to do a sneak attack if not for the warrior (or anyone else) standing next to him. So the warrior should get some credit for that damage in your group, too. It's a tactical decision. Sure, the guy with the football in the endzone get's the photos and headlines, but in the end, it's the team that wins or loses, not the single player.
The Alert feat (PHB, p. 165) states that:
Note: This doesn't stop Swashbucklers from performing Sneak Attack on you or if a Rogue of any archetype has an ally nearby.
Heheheh…there is a way. The Wizard Abjurer's Arcane Ward is a seperate entity that surrounds and takes damage for the Wizard. It is also an inanimate object, the only thing immune to a sneak attack. As far as I know, that is the only way to stop a sneak attack other than a quick casting of the Shield spell to bump your AC over their attack roll, which does not always work. You could put yourself in a metal box, another inanimate object, which grants immunity to sneak attacks but also…you are rather trapped in a metal box.