It really sucks that your family isn't more supportive
I would sit down calmly with your mom and talk to her about it without being upset. She should be able to buy whatever she wants but you could maybe help plan out some healthy meals and offer to cook them for the family and ask her that if she buys junk food that it will be put out of sight, like in a high cabinet for all the junk food or something like that. And then you just have to stay away from there.
Temptation will always be in your life. Even when you move out and are on your own going to the store you will be confronted with all the bad choices, going out to eat with family/friends, work, get togethers, etc... So the best advice I can give you is to learn to deal with the temptation now. It is still your choice if you choose to eat it or not.
Try to work out a good system, when you go to grab the chips think oh those are not for me, i want to save them for dad or something. Always have a backup healtheir snack you can easily grab when temptation hits. Or come chat on the forums that helps me alot lol.
I know how hard it is. It is just me and my husband but he eats whatever he wants. We eat all our meals seperate lol. He still buys chips and ice cream and whatever he wants. I just tell myself those are for him only. Though sometimes it can be really hard to resist. Those times I will steal just one cheeto and walk away or have one ice cream sandwhich that week and not touch the box again.
You can do it!!