This is an old thread I know, but I've had a slightly similar problem - but I was the problem, not other people.
The hardest thing to do I found was to find the motivation or desire to get up & move, I didn't care about the effect my laziness was causing my health & fitness until I saw a program on Foxtel that made me finally wake-up & move.
If you really care about the people that your worried about, tell them that you actually are worried about them & point out reasons for your concern.
Just telling someone they are fat would hinder rather than help - although it may be true!!
A little bit of tact & decorum goes a long way & genuine concern.
Maybe suggest doing some active things together, getting out of the house - go bowling, ice-skating, walk the tan - there are heaps of things that you can suggest to get active. A simple thing like Summer is only a couple of months away can be a wake-up call.
You can't change bad habits overnight, but if you have the persistence, I'm sure you can make a difference. You obviously care for these people & they are lucky to have you - some people don't have that luxury.
Hope that helps.