If he won't do it for himself, you or his wife, what about his daughter. You say he takes care of her correct? What if she was running through the house with a knife she quickly grabbed? Is he able to catch her and keep her safe? What if they are outside and she darts for the street, can he catch her before it's too late? So who is he hurting here?
Do you work? Do you have kids? Can you ever get him and your Niece to go to the park or somewhere with you to play? Just you guys, no wife? You should know his "buttons" you're his sister. Start talking about the time he played football and baseball, wouldn't it be great to play softball? I have two sons, one is 30 the other is 34 and they both play softball, the oldest one has 2 kids of his own and helps coach them in sports. My Granddaughter is awesome in soccer, what is his daughter going to want to play? Will he be around to help her? Or die of a heart attack?
I think, if you can't get him out with his little girl, then maybe you need to do two things, one - if you have a decent relationship with his wife, talk to her.....explain your concern for his life. 2- you two go out just a brother/sister thing and really talk to him about how and why you feel the way you do. How much you love him, how much you want him to be around for you and his family.
You might be able to do an intervention, if your other family members feel the same. Get some facts together to show him what a man of his age and height should weigh, then ask him how "little" over is he? Show him the chances of heart attacks. Offer to walk with him, run with him, whatever you can do to get him to start moving......before it's too late.
You're a great sister. Hang in there, he's lucky to have someone who loves him like you do.
Take care.