Familiars cannot attack, but they can provide contact damage. I want to know if the rogue's sneak attack works for their familiar too. (Granted that the familiar has an advantage on the target or an ally (the rogue) is 5ft away.)
I think you're asking if a familiar can perform a sneak attack. The answer, is No. There are two reasons for this.
- Sneak attack is a rogue class feature. A familiar is not a rogue, and does not have rogue class levels, thus it doesn't have access to this feature. Even if you have rogue class levels, those levels do not apply to your familiar.
According to the sneak attack feature (PHB 96), to make a sneak attack
The attack must use a finesse or a ranged weapon.
A familiar cannot wield a weapon, so it cannot make a sneak attack.
Now, if you're asking if a familiar can grant your rogue the ability to use sneak attack by being within 5 ft of the rogue's target, I would suggest that as long as your familiar can cause damage, the target must be guarded against it, and that split attention grants your rogue the ability to make a sneak attack. But that interpretation may be up to your DM.
A familiar can perform a sneak attack - if for whatever reason they have a sneak attack ability of their own. The familiar can't use the rogue's sneak attack ability because it's not the rogue attacking.