Looks like a good start, but i think you need to work on your curve. If you look at your mana curve it isnt very smooth. You have a bunch of cheap cards but then it has some outliers namely Desolation Twin. It has a pretty big impact on the board but without much in the way of ramp it seems unreliable to cast. I could see it being stranded in your hand more often than not. If you want to keep the realtively cheap curve or be midrange i would cut the twin. If you want to be able to reliably cast him, you would need to add significantly more ramp. Emrakul, the Promised End has a similar issue but due to the delerium ability it might be more reasonable. With only instants creatures and sorcieris (I don't forsee lands adding to delerium easily at present), she will at cheapest be 10 mana as well. If you added cards like Warped Landscape, Evolving Wilds, Terramorphic Expanse, Polluted Delta etc you could reliably get another card type in the yard

Speaking of lands, 2 more Eldrazi Temple and a second Eye of Ugin (if this is out of budget don't worry about it) could also help cast eldrazi

Yu might also consider having a few lands that can produce colorless. If in addition to the temples you have 4x Underground River you could reasonable play Matter Reshaper, Reality Smasher or Thought-Knot Seer

oh also if you want to be mean, Night of Souls' Betrayal and Dread of Night are sideboard cards that make it so soul sister really struggles

Some nice flavors here! What type of sideboard do you run?