I'm sorry your sister is going through so much.

I would recommend PsychEducation | Treating the Mood Spectrum to help learn about bipolar. It is written by a psychiatrist who specializes in bipolar and has lots of helpful information.

Bipolar usually starts in the early-mid twenties but plenty of people on here were diagnosed much later.

I have never had imaging done and don't know anyone who has but it has always seemed logical to me so maybe it is worth suggesting your BIL bring it up to the psychiatrist if he is comfortable. The worst thing that happens is they say no.

Welcome to PC.