[ Family ] Open Question : How should I handle this situation?

This may be kind of long but I would really appreciate some opinions on this. Here's some background information. For most of my life I've been raised by my mother. I have 1 older brother, 1 older sister and 1 younger brother. My older siblings are in their 20s and still living at home but I feel that all the responsibility regarding house work rests entirely on my shoulders. I know that's true because my mom tells me that I'm the only one she can depend on. My mom works from about 8 am in the morning until it's past midnight at her job (she's self-employed). I don't want everything to go to hell while she's working so I do my best to take care of everything. Now I'm going to go into the real issue here. Being that my mom has always been working, she never learned how to cook. So I've basically been raised on fast food and tv dinners. I never really developed much more than a small pudge on my stomach but my mom is overweight and my younger brother was a bit out of shape too. I didn't want them to have to keep eating garbage so I learned how to cook when I got out of school for the summer and everyone says I've gotten pretty good at it (even my older siblings approve). I've also managed to leave the junk food alone and really start working out (for the purpose of getting better at a sport that I'm very passionate about). As a result of that, I am now in the best shape of my life. The problem is that I'm about to go back to school and I'm also going to be working. I want to start cooking even healthier but my mom annd my brother aren't really on board with it. I've been cooking semi-healthy (with some fat content) for them and because I've had time I've also been able to make healthy stuff for myself. I won't have time to cook semi-healthy for them and then cook even healthier stuff for myself so that I can maintain my good shape. My mom tells me that she's trying to lose weight but she can't leave the junk food (she still eats chips and other things) alone and really commit. Should I keep pushing the healthy stuff or should I just keep making what I know they'll like so they won't go back to fast food?