I pretty much agree with all of those possibilities that Katie offered....save one. I think Olga knows her brother well enough to know that if she poisons one of them, or if a drug she gives Ingrid causes her to miscarry....sister or not, he WILL kill her. In order to guarantee her survival, she would need to get all four of them at once with the poison. And different size people, different metabolisms....there would be no guarantee she could. Which is why I think she'd stay away from that one. But using Ingrid's weakness and Angelique's need against Alexei? She'd be TOTALLY in for that. Or Allying herself with Brogin....we all know she'd ally herself with the devil himself if it got her power :P