I Am 13 And My Little Brother Is 11. We've

I am 13 and my little brother is 11. We've never had the best relationship, we used to fight constantly but now that we're older I've been trying to be a better big sister to him. Our other little brother is autistic and takes up a lot of our parents time. My dad sometimes yells at us unprovoked due to high stress levels. He's been especially hard on my brother recently. I've noticed that my brother seems to harbor a lot of feeling of guilt weakness and seems to be insecure. Like I said we've never really been very close so he doesn't really trust me. I've noticed that he lies about eating meals. He says he's eaten lunch or breakfast when I know for a fact he hasn't and gets mad when I bring it up. I'm worried about him, and don't want him to get hurt or make decisions he might regret.