I would like to ask for some advice about my unrighteousness. I hope you could reply.
To get straight to it, I had sex with my brother. It was not really the kind of serious kind of sex because we were just playing around. At that time we were already in the church for two years. I don't really know why I did it. but now I regret all of it. I'm afraid that God may not forgive me and my brother.
I seek your advice about my situation.
All sins can be forgiven, but forgiveness is based on a person turning away from his sin and changing his mind about the acceptability of sin. This is called "repentance." "The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance" (II Peter 3:9).
You regret the sin you've committed and that is a strong motivation toward actually changing. But the sorrow is not the same as committing to changing. "For the sorrow that is according to the will of God produces a repentance without regret, leading to salvation, but the sorrow of the world produces death. For behold what earnestness this very thing, this godly sorrow, has produced in you: what vindication of yourselves, what indignation, what fear, what longing, what zeal, what avenging of wrong! In everything you demonstrated yourselves to be innocent in the matter" (II Corinthians 7:10-11). I have no idea if your brother even regrets what he did, let alone repent of his sin.
One of the things you have to face is that all sin is serious and cannot be excused. You understand that what you did was wrong, but you attempted to soften the matter by saying it wasn't serious sex since you were just playing around. However, it was still fornication. It would still keep you out of heaven if you don't change and ask God for forgiveness (I Corinthians 6:9-10; I John 1:9). While we tend to see fornication with a family member as a worse sin, from God's view it is still the same sin.