How do I tell my boyfriend about the abuse?

Awe sweetie, this is NOT your fault, you are the child, and that brute should know better. If you cant tell your friend then go tell another family member, or speak to someone at school. If this carries on someone, namely you or your sister are going to get very badly hurt, if you are not already. Explain to them what you have said here. You must be scared out your wits. PLEASE go and speak to somebody, or call a help-line if you can. Will you wait until one of these brutes kills your sister, think about it honey. GET HELP NOW. What about calling your parents, maybe they can come home eh!