I think the dad, who has the best visibility of the situation, gave you the best advice.
However, this doesn't have to be approached as a "I think you should do this" type of scenario and it doesn't need to be as direct and confrontational as a phone call.
What about a card, offering your best wishes on the situation with a note that you are concerned about the daughter and with the information, factual, not opinion, on what you see happening with the daughter. You could also offer to help, even so much as to bring the daughter.
These parents have a great deal to deal with already and the mom may be so focused on what is going on that she isn't seeing or thinking about fallout with other kids.
The kids don't need to see the brother if that is too hard. The parents don't need your opinion or condemnation, just information about what YOU are seeing that they can't see. The the adults in their family will make decisions about what to do / how to handle it.