I actually find this thread to be really quite interesting. Obviously we none of us know the OP or the person with MS, so ultimately our opinion is irrelevant.
The thing that I find interesting is that communication has hardly been mentioned. A few of us made knee jerk reactions. It seemed as though OP was trying to dictate the appropriate behaviour of the PWMS. This may, or may not, be the case.
But it appears that the whole subject has been told to us in a rather confrontational and bossy manner. This could be what the PWMS is reacting to. Maybe if the subject had been raised in a more loving, caring, ‘us against the world’ way, there could have been less negativity and argument.
Clearly it’s a bit late now, but if the OP approached the whole business in a more loving, caring way, perhaps it wouldn’t have degenerated into a row.
If you suggest perhaps ‘we could all be fitter’, ‘maybe we could go on a diet?’ ‘perhaps I could help you with exercise?’ it might not have got so heated?
Clearly you have your relatives best interests at heart, so perhaps those of us who initially decried you as being a bully should think about the other responses (Ellie, Whammel, Reddivine, Alison) and consider that you yourself feel put upon / bullied. The fact is that it’s not a happy or healthy way to live, either for the PWMS or you.
I do hope you and the relative with MS find a way to heal your relationship and hopefully to persuade him to try and do more physically and with regard to diet to help him have a fitter and more controlled relationship, with his body and MS.