Hey rise_up,
Sometimes upholding your boundaries can be exhausting. You've made it clear to everyone that an August visit just isn't going to work for you. You've been very consistent about that.

It's very likely that your mother is pressuring your father and your brother to push you to change your mind, and I can imagine why that makes you angry.
Could you take care of your visa papers? That might give your parents less control over you. Why do you think they're so intent on having you visit from August to December?
You're doing the right thing by being consistent. Is it worth reaching out to your parents again about visiting in October?