I don't think we want to try beat UW control on the counterspell axis, we're not the kind of deck they remove their counterspells for, so we're likely losing the counter war, especially trying to fight a 1 mana miracle Terminus. I personally feel we have two viable paths to fight them on, the get in under them aggro plan if you're more a white weenie beat down plan, or the out grind them with value plan, this one is slightly harder given their card draw and card selection with things like azcanta, but not impossible. Having multiple must answer threats like Hero of Bladehold, archangel of thune or even sun titan and cards for some advantage like Ranger of eos in the matyr proc lists or Legion's Landing and Windbrisk heights in a more sisters midrange package. I've personally lightly splashed black for Lingering Souls and Sorin, Solemn Visitor for further value since I'm more on the midrange game plan.
The fact it makes 1/1 lifelinkers for 4 means it might not be bad in the more grindy matchups, I mean legion's landing has been doing a great job of that for me.
http://tappedout.net/mtg-decks/28-03-18-soul-sisters/ This is my current list, It is slanted to be more aggressive and get in with fliers Getting in an archangel off windbrisk heights is often a back breaker, but even hard casting is great. For some example of gameplay and how aggressive it can be
I like the list. Are you going to replace the heroes with leonin warleader? Also why don't you have any anthems like honor of the pure or benalish marshal?
About the sideboard, why relic instead of rest in peace?