I have to strongly agree with some of the other posts and point out to you that you seem to be very focused on what you and your family look like. You did not mention if your sister was happy or if she was a good mom or if she was married, or if she is at a healthy weight (is she 140 pounds and you call that huge because she's always been 110 or is she 300 pounds?). I read your post feeling like you have no respect for your sister just because she weighs more then you feel is appropriate.
You have a lot of concern about your brother not eating and wanting to help him is great but before you can do that perhaps you need to look at yourself and your family to see how you react to each other. Why did you feel the need to tell us in your post that you have no stretch marks? you should be proud of your body but you can't expect everyone to go through a pregancy or two as easily as you. Have you ever expressed concern for your sister and her weight issues? Maybe you could offer to take walks together or make family dinners together to help her with her food choices?
I'm not trying to be hard on you but I was a normal sized child (looking back at pictures I'd even say skinny) but after I "developed" I was much curvier then my sisters were and my sister's often teased me and called me fat or cow or whatever. I know now that they were just trying to push my buttons and they didn't mean it but it did affect me and my eating habits. I am VERY careful about calling me daughter skinny or fat or anything like that. We talk about building muscles and being strong but never about her size (she's very normal). She's only 4 years old but I'm extremely aware of how those kinds of comments can stay with a young person. You should look inside yourself and think about how your comments can affect your own children and your family.
As for your brother, tell him to let you know when he wants help but be very clear that what he is doing will damage him for life. He needs to want help or it won't work.