Your dad's hitting you any time he's out of control (drunk, angry, whatever) is abuse. Hitting you with a belt or rope hard enough to cause bruising probably would qualify as abuse also, depending on where you live.

Any idea why he's singled you & your twin out? Have you ever talked with your mom about this?

You need to talk with someone you trust--a teacher, counselor, sports or religious person, neighbor, friend's parent, doctor. Take your twin with you for support. Your dad needs help too. If he drinks a lot then alcohol may explain the degree of the beatings.

Please don't keep this a secret any longer, RubenRawr. You or your brother could be seriously injured if he gets drunk enough.

PS
Now with what you've added, this takes on a whole other dimension. More than ever, the secrets must end.

Please take this to whichever adult in your life you most trust. Do it now, however scary. It is scary, but going on as you are could turn out much scarier.