This deck is actually very cool, +1'd it

I think that the Ghost Quarter should be left out, the colorless mana will slow you down a lot. Only 2 Windbrisk Heights are enough, it will be hard to trigger the ability off of it if you don't have Spectral Procession plus you don't have much costly spells to hideaway to be really worth having land that comes in the battlefield tapped. Same thing for the Mistveil Plains, I would put only 1 or 2 in to keep as many Plains as possible. Casting a Martyr of Sands turn 1, turn 2 sacrificing it to trigger + cast Serra Ascendant to attack with a beast 6/6 flying lifelink, you need to be quick turn 1 because this is what the deck is all about.

Otherwise, this deck seems pretty beast

HockeyprO thanks! I changed the landbase

I wouldn't be playing Vexing Devil. He doesn't fit the deck and wastes valuable card space.

This is such a cool twist on Soul Sisters +1! Makes me wonder what other colors one can mix with this mostly mono white strategy...

ledvulture I wouldn't mix with any colours. Mono W is the best for this deck.

That said, if you do splash, run about 4-6 white fetches.

I really like this deck idea, but personally, I think Mono-White is the best for this deck, as you can much more consistently cast Spectral Procession and your martyrs are a lot better. Also, I think it is worth playing the full eight "soul sisters." You can check out my soul sisters deck with this link: Soul Sisters Modern Deck.

Take Squadron Hawk out of the sideboard. It is a waste of slots. You want answers, not extra cards.