I Walked in During Her Morning Jerk off and We End up Fucking!

I am staying at my bro's place right now for a few weeks. He has been so kind with me, and his wife too! But everything started being confusing after that morning I left work really early and heard a little moaning when I came home... it was my sis-in-law masturbating... I can't describe how hard my dick was when I heard her soft moaning and watched her in her bed touching her pussy...