How do you tell someone you love that they are too fat?

It’s a hard one I was near on 17 stone at got down to 11 a few years I’ve put a stone and half on since the fiancée moved in a year ago but I thought maybe seeing how much happier I was slimmer it may off given her a kick up the arse but nope. My sister is very much a hypochondriac she likes the attention and is always moping around it doesn’t help that her husband has piled the weight on to there both stuck in there ways and I’m ashamed to say a couple of bums, we were well bought up but had nothin mum and dad worked there arse off and are comfortable now but if I ever wanted nice stuff that my mates had I had to go and get it when she was in her teens mum and dad were better off and she had what she wanted, I think you go two ways I had nothing and wanted everything so I started my first job when I was at school she on the other hand sees it that she doesn’t miss what she’s never had and is happy to bum through life with not alot. I can’t understand the mentality off that and it gets up my nose cause I want her to have a bit off drive and ambition her weight is getting worse and I don’t know if its a cry for help or her way off gettin a bit more attention but I find myself smacking my head against a brick wall as I said I think she wants to loose it but she has to donit on her own and she’s not used to having to work hard herself to get something she thinks me working the hours i work is stupid i see it as life that’s what I have to do to pay a mortgage have nice cars etc that’s the difference between us I am going to be brutely honest with her though and it may not go down well but nothing else works she needs grabbing hold of and shaking into the real world lol it may harm things with your sister to be honest with her and tell her but I’m thinking once she sees it and looses some weight she may thank you for being the one who kicked her up the arse I’m gonna try it and just hope it doesn’t back fire but its not nice sitting back and watching it happen.