My mum died in January this year. She was in a psychiactric hospital for much of my teen years (also bipolar) and was also a drinker, a drug user and had several abusive boyfriends through my childhood.
she was also a talented poet and influenced me creatively.
I have talked to my T about grieving not only for the mother I had, but for who she might have been, who she could or should have been if it wasn't for her illness.
It sounds like you might be feeling in a similar way about your brother? I also feel my mum would still be alive if she hadn't mistreated herself.
Hugs and empathy