My daughter is becoming obese.

You could broach it in a straight forward and kind way. Tell her you want to help her straight up. Sometimes people need to hear the truth. Perhaps suggest a gym membership, you could go together. Or suggest you go on a healthy eating plan together. Edit. I don’t see why a father talking to his daughter about her weight in a kind and constructive manner is an issue. As a mother I would talk to my son or daughter if that’s the case. She might not like hearing it, obviously she is aware of what’s happening. She may be stuck in a hole of depression. Your wife is Sick and is obviously dealing with with that. I would suggest that it’s your duty to broach the subject with her, as her dad.