At this point, I feel it's a throuple thing. Dasha and Sasha seem to spend equal time with her, they trade nights in her bed and they have nights together in their own bed. They also spend time all together, whether it be meals, TV or threesomes.
I don't foresee a 'breakup' between Dasha and Sasha, only Nancy possibly moving on. Unless I'm right about a prediction I made a couple weeks ago and they breed Nancy because Dasha can't have children. Only time will tell.