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MissPerfect Yes. But if they are perceptive enough to have noticed you won't be telling them anything they don't already know. If they aren't do you to think that knowing will prompt them to do anything about it?
I mean to say you are right to be concerned for her but saving her from the situation she is in is likely to be a big and contentious project.
I have first hand experience with an overweight family member (not even close to that extreme) and it was a very difficult topic to discuss and ultimately futile.
So, don't feel too bad about it.