When he makes cracks about your sister’s weight, tell him as politely as you can, “please don’t say those kinds of things about my sister. Even if they don’t bother her, they bother me and they are unnecessary.” Keep repeating this. If nothing else, he will get sick of hearing you say this and stop. If he persists, then you may have to get a little mean. Be firm. Say how it’s not up to him to worry about her weight. As for when he tries to do something better, again, be polite but firm. Say, “thanks, but I’d prefer to do this myself,” or “I need to do this a specific way and it’s easier for me to do it myself than to explain how to do it.”
Just out of curiosity, how old are you? Unfortunately, the younger you are, the less power you have in situations like this. I would also suggest talking to your mom if you haven’t already.