Karen — a 12-year-old girl
Susan — also a 12-year-old girl, Karen's longtime best friend
Jane — also a 12-year-old girl, Karen's best friend of 2 years
John — a 12-year-old boy, Karen's ex-boyfriend (they dated for about three months), now the close friend of Karen, Susan, and Jane
Karen has a mutual crush with Susan. However, Karen's also had a fairly minor crush on both Jane and Susan ever since she broke up with John — actually, Karen had a pretty minor crush on Jane even before she broke up with John (that was one of the reasons she ended their relationship).
Susan (who had come out as bisexual a few weeks prior to Karen's ending the relationship) told Karen, about two weeks after Karen had become newly single and had officially come out as bisexual, that she had a crush on Karen. Karen admitted to having a crush on Susan (which had developed about three days before Susan told Karen she liked her), and thus their mutual crush began.
When Karen told Susan she liked her back, she also revealed to Susan that she kind of liked Jane as well, so Susan's known that Karen likes Jane too for a while. (This creates fewer complications in their overall "relationship", if you can call it that.)
Karen's crush on Jane has kind of expanded and grown since about two weeks ago. Karen went on a trip to an amusement park with her and part of their school's Orchestra and Band. They sat together on the hour-and-a-half-each-way bus ride, and Jane fell asleep—leaning on Karen's shoulder—for about half an hour on the way home (it was nine p. m.). That's pretty much where Karen's crush on Jane peaked, and now it's kind of still at that same level of climax.
*DO NOT OFFER ADVICE FOR KAREN YET! There is another part of this question coming. There's just too much text for it all to fit in one question.*
Part Two— What would you do in this situation? ↗