One of the most reliable and insightful guides in the world is the heart of a woman--especially if she is a woman of prayer. Today I met a dad who was struggling with looking at on-line porn. At the same time, he was a youth minister and a husband, but nobody knew of his addiction. One day in prayer he said, "God, I really want to deepen my intimacy with you." Two days later, a woman at his church walked up to him out of the blue and said, "I've got a word from the Lord for you. If you want intimacy with God, you need to throw away your pornography. You have to make a decision between God and the porn." Wham! There is absolutely no way this woman could have known what he did in the privacy of his own home. He was immediately cut to the heart, and broke his habit off, and asked forgiveness from his family.
In your case, your heart is speaking the truth to you. It's saying, "Listen to me. You are being used. Get out now." You must listen to your heart, because when a girl is really being loved, cherished, and led to God, she never, EVER, wonders if she is being used. When that "Getting used" alarm clock goes off in a woman's intuition, she better act on it fast, before worse things happen. Unfortunately, the world tells women to doubt their intuitions.
You have about five warning signals that go off for me, saying that this relationship is bad news:
1. You said you "kind of have a boyfriend." These are not boyfriends.
2. You said you had to hide it from your family. The healthiest relationships in the world are those in which you are thrilled to have your family know the full truth, because your family loves you more than any guy. When you need to hide a relationship, it's because it is unhealthy or harmful for you. If it weren't, then there would be no need to deceive those who love you. If you ever have to conceal a relationship, or to try to convince your family to approve of a guy, something is wrong.
3. He's your step mom's half-brother. I would guess he's a little older than you, too, but even if he's not, you really don't want to do sexual stuff with family members. After all, this is not the kind of stuff you want people to be finding out about. So, end it now. In the future, don't do anything in private unless you're comfortable with it becoming public.
4. He's done sexual stuff to you. This is not love. It is lust. He's not leading you closer to God or guarding your innocence. He's wearing it down.
5. You suspect you are being used. You are right.
So, get out of the relationship, and don't put yourself in situations where physical stuff can happen with him. I hear of so many rapes that happen from step-family members, and I would never want you to be in a situation where that could happen. Most of the girls who it happens to trusted the guy (before it happened). Talk to someone who knows about the situation, and ask their help in getting away from him, and staying away. That way, you have someone who you can be accountable to. In the meantime, pray for strength, wisdom, healing, and courage. And if you have not already, get to confession because you receive an ocean of graces there. Until then, trust your heart!