Sorry to hear you're going through this. I agree it must be hard to watch the ripple effect and know there is nothing you can do about it.
You can't make her have sex with him and that is going to cause a ripple that rocks your boat. It is highly possible its a power play. It is also possible her trust has been broken and she has chosen to not have sex with someone she can't trust. Its hard for me as a reader to know either way; lack of detail and all.
I do caution you on two points though. Unless you know for sure she snooped, you might be accusing her of something she didn't do because you're, of course and who wouldn't be, hurt and angry. I get it but it might be that he told her things and just isn't being up front with you about that. You don't say what this things are that you feel she knows.
The other is they chose a path that resulted in him ending things with you. For good or bad, this usually happens when the other relationship seems threatened or one of the partners is going outside agreed boundaries especially if it seems the person they are seeing encourages this. Encouragement like "lets do it one more time anyway" is only one example. I don't know the whole back story but make sure you're not being that encouraging to push past agreements kind of partner. If you are going to be that person, why this is happening might not be so hard to understand.