Sneak attack needs to have advantage on attack except in these circumstances:
Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.
You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll
I am struggling with understanding the "isn't incapacitated" requirement, because from my point of view it's the best time to strike. An enemy has no chance defending himself while being incapacitated.
Best Answer
This is an understandable misreading. "that enemy" refers to an enemy of the creature being targeted with the attack.
In other words, to trigger sneak attack, you need someone within 5 ft of the target, that other person also needs to be an enemy of the target and can't be incapacitated.
The in-game interpretation of sneak attack is that the rogue is exploiting an opening or distraction during a fight. Having another enemy standing next to you is certainly distracting but not so much if he is incapacitated.