That sounds like a horrible idea, imo. She has type 2 diabetes and she's overweight. It's not like she doesn't know she has a problem. Some of the medications used to treat bipolar cause massive weight gain.

Maybe the doctor treating her for the diabetes could suggest some sort of support group for diabetes or weight loss? There are just so many other possible solutions to this problem that are less hurtful than a brother confronting a sister about her weight.