I'm torn. On the one hand, I agree with WillyNilly. Except in extreme cases, I think people should be up-front about problems as soon as they come up. For example, did anyone frankly tell this woman she was abusing her privileges and that it was causing resentment/tension? Has anyone told her that not reciprocating invites makes people want to stop inviting her? Note, I wouldn't do this with an acquaintance. But someone who has been a friend for years and years? She needs to know when she's doing something that's causing her to lose friends. And if no one has given her the chance to change, just quietly fumed about it, then I think she deserves to know what people have been thinking.
On the other hand - this isn't your event. It isn't your place to tell her why someone else didn't invite her, it's up to the person who didn't invite her.
So, I suggest a combination of the two. Say something like, "Listen, you have to ask Sally directly why she didn't invite you. I don't want to speak for her."
And then, in a different conversation, talk to her about the problems that you've been having with her. It doesn't seem fair that she never reciprocates, it doesn't seem she's a good fit with you anymore. That way, when it is your turn to host, she already knows why she's not invited and she won't put someone else in the uncomfortable position of having to fact this same dilemma.