Do we need another Soul Sister?

Glad you brought this up, because I don't really want to name it either one of those names because I feel like they imply that he should do something explicitly with the spirit tokens rather than doing something more subtle. This could change. The fact that the card is in the set probably won't change, I think it's a really strong card for the format. Also, as the days of the Soul Sisters deck are long gone in standard, I'm not biting my nails on this one. If it would push Modern over the top, whatever, I guess I could just do Soul Warden or something.

­Soul Warden is indeed very strong in multiplayer; unfortunately, Soul's Attendant rather less so, for precisely the same reason that she exists at all. There are three of them (Essence Warden) and that's not caused much problem, so I guess it's probably not a problem to have a fourth.

Not sure what you mean with the comparison between Souls, but Attendant is an optional, so it's a little worse than the Warden. The thing that may push a deck over the edge would be the soul sisters deck in Modern, but I really think that it'd be okay, Inkmoth Nexus is all over the place.