The chances are very small, and probably no greater than random, especially since the diagnosis of bipolar has many levels, and often many errors.
It seems like the sister's diagnosis is pretty convincing (I am not a psychiatrist, but I am a physician and I have spoken with her psychiatrist, and personally observed her in a manic state with psychotic features, alternating with nearly catatonic depression). Does this increase the odds, or would you say they are still low and close to random? Thanks.