(Preface: since I made this answer, I reviewed my thought and added a separate answer that I think is more accurate, since its conclusion is different from the accepted one, I left it as it is).
Purely RAW Response *: Attack rolls are not clear, but damage rolls must be a single roll for all targets (including Witch Bolt tracking actions for 1d12 damage per round).
Basic rules of D & D, p.75:
If a spell or other effect deals damage to more than one target at the same time, shoot the damage once for all of them. For example, when a magician throws a fireball or a cleric launches a fiery blow, the damage of the spell is cast once for all creatures trapped in the explosion.
Sister witch stud is still a single spell instance, you have just changed your orientation mode from 1 to 2 objectives. As such, it is a spell that inflicts damage to two targets at the same time (assuming it hits both), and without a more specific rule to defeat the previous general, it should apply. If I lack a more specific decision that says a single damage roll per attack roll, it is possible that part or all of this does not apply, but I have not seen such a precise statement in a quick glance.
As for the attack rolls, I can not find a specific RAW fault on that. I believe that all AOE items with attack rolls have an initial single attack roll followed by an AOE save around the target, or they are spells / effects that have a specific decision on built-in attack rolls such as the Eldritch Blast ( beam-separated) and The Battle Master Fighter's sweep attack (use the original attack roll for the second target).
As such, there is no "general" rule in multiple target attack rolls you can find, since the twin spell seems to be the only way to cause one that does not have its own built-in specific rule.
* The RAW interpretation of this is presented as a bit weird, especially because there is a gap in the rules where there is no general rule about the AOE attack rolls that I can find. Certainly, I would not have a problem with a fault & # 39; made at home & # 39; to fill the gap, but I wanted to provide everything I can find in the RAW.