If they're happy with each other, it's not for you to judge.. and you are judging by suggesting he's acting like her child. I'm sure he's aware about his previous weight-loss accomplishment and "falling off the wagon".. no need to point out the obvious.

If your sister-in-law has not reached out to your wife (her sister) then either they're managing things just fine for "them" or, you're simply not the "go-to" couple for their support.

I'd leave it be and allow them to open that conversation with you, instead, if they're even willing to. In the meantime, accept them for who they are and love them as you do.. IMO