Those are great questions. If there was an issue with sexual abuse, there is definitely a possibility of her not saying anything, until now. Sometimes when children begin to speak about it, it can mean that the abuser is no longer in the picture and she feels more safe to speak about her feelings and her experiences. Or, it could be something that triggered her to speak about it, like nightmares
If she is speaking about the sleepover that she attended, there could possibly be something more that has happened. With children, especially if there are feelings of guilt, they will want to speak only a few things about what has happened, to make sure it is "safe" for them to tell more.
Usually in children, they are not normally diagnosed with bipolar, at such a young age. But, she could definitely be diagnosed with depression, if she is exhibiting depressive symptoms. Usually a bipolar diagnosis does not happen, until a child is in their teens, but having a mother with bipolar, a diagnosis may come earlier, if she is presenting bipolar symptoms.
Ruminating about her experiences and having guilt, does sound to me, to be more depressive in nature.
Sometimes, when children are exposed to an incident that is sexual in nature, they will re enact that sexual behavior with others. It may be worth letting her know that this is normal for someone who has experienced things as she has. This may also make her feel more safe in telling more of the story, if there is one.
I do hope this helps and please let me know if there is any other questions you have. I would be happy to answer them.