Last time I checked, humans are required to eat food to live. So here is my advice:
Next time you want to keep yourself alive(aka, eat some food), and you go and get some food, and they complain? Tell them, tell them you are not going to take their shit, and you WILL eat.
You tell them, how this makes you feel, how much it pisses you off. Do not take shit from anyone, not from faggot step dad, nor his faggot son, not even your very own flesh and blood mom. The fact she can treat you that way, over those two people sickens me. And something needs to be done. I am pretty sure there's some kind of law that punishes those who abuse the disabled, which they really are doing to you. It's sickening, seriously sickening. So probably try looking in to that?
Also, if I were in this situation, I would handle it in a way most would find extreme(do keep in mind this is not a joke, and I recommend you to NOT try it):
I would snap pictures of myself in the nude, crying immensely, with what appears to be the cameraman holding a knife in view of the camera to me.
I would then take those pictures and stash them anywhere my mom could find them easily, and then hilarity ensues as the dad is accused. But I recommend nobody to ever try this, unless the situation is really fucked up.
(and if anyone is wondering why I even threw that out there, it's just an option for when things like I said, get really fucked up and you need revenge)