I can't take my mum much more!?

I would not want to eat food someone has played with. There is definitely a mental issue going on with your sister and mother. There is a power struggle for whatever reason going on between your sister and mother. And since your mom can't win she tries to use her power over you to make you eat the food. Your mother needs to take your sister to the doctor about here eating issue, because it is not healthy and it is causing a problem in your family. And you need to find someone to talk to about this a family doctor, school counselor, or a friend you can trust. I hope you will be ok through all of this. Hang in there.