Even if you are family it is still inappropriate to hassle someone about their body. That means if you really care for someone badgering or belittling them is not okay. If you couch your harassment in pseudo-caring terms you are harming that person.
Let me repeat that so we get it.
If you are insulting, back hand complimenting, belittling, making fun of, poking fun at, even doing that passive aggressive not coming out and saying it thing, playing concerned for the persons health in a way that is little more than shame wrapped up in bullshit, you are hurting that person.
Ask any person who was a fat teenager with Moms especially who engaged in this kind of behavior, if you ask them honestly they were hurt. Some hurt so badly that their self esteem never recovers.
If you think you are acting out of love and you are only causing harm even if the person you are harming never says anything you think about it. If the person you are talking to this way tends to cower, if they get angry if they seem to just kind of start disappearing you need to know that you are responsible for that.
If you really care for and love your fat family member or loved one, you will not participate in their shaming. You will learn to support that person and help them build the self esteem so they want to care for themselves. You will not judge them because their body isn’t the body you want. You will celebrate that person when they come out with high self esteem and a good sense of self.
That is part of your role.
And if you cannot bring yourself to do that for whatever reason at least have the decency to leave the person alone about their body.