I'm sorry for everything that's been going on with you but you seem to be holding up okay which is good! Are you worried that your friend won't write back? If he's been through similar things and is your friend, I wouldn't worry because he surely cares about you and will want you to know that he's there for you! What are you so nervous about if he finds out? You seem to know him very well and he seems like a trustworthy man. If you told him not to tell anyone about what happened, I doubt that he would go against your wishes.
I'm curious though.. why do you not want your friend to tell anyone about your father who has abused your sister and stepmom? Abuse is a very serious matter. Hurting someone you love is just as serious as going up to an innocent person on the street and breaking their arm. But aside from that, the fact remains that he did it about a year ago so there wouldn't be too much of a case because the physical damages are probably repaired by now. However, if your father abuses someone again, I think you should call the police on him. I know it's difficult because you love, but he's hurting the people around him and they may be more dominated and influenced by him than you in this situation.
Don't worry though. Your friend seems like a great guy and he will most likely let you make the decision regarding your father and his unlawful behavior. If you enjoy sending letters to him and trust him fully, you should definitely keep up with it!