Hi, I'm 15

I would tell her, because a lot of people with bipolar inherited it, so It may be good to have her alerted, I think she would understand. Just tell her that you are bipolar, which means you have bad mood swings that you can't control. Like, you get sad for no reason, and then you get happy for no reason, and sometimes It gets out of control. I would stay away from the term mental illness, because it just carries a bad stigma. If she is wondering about the therapist and meds, tell her that doctors think the reason you get the mood swings is because a person's brain normally releases chemicals that make you happy when good things happen, and the chemical levels go way down when bad things happen, and for some reason your brain doesn't control the chemicals right, and randomly sends out the chemicals, or randomally hold back the chemicals, so the meds help control the chemicals, and the therapist is to make sure the meds are working right, and so that you have someone to talk to about everything, someone that can help you deal, and adjust your meds if they think a change is needed to better serve you.

I hope that helps.
12 year olds are smarter then ya think