I (M30) went to check on my (F28) sister after she had a miscarriage and found her husband (M29) in her bed, what now?

So my sister and her husband have been married for 3 years now and she recently found out she was pregnant. 2 weeks ago she found out she miscarried and my parents and I were obviously really upset. You can imagine how bad it must be for the couple. <br><br>But they said she was feeling really bad and asked if I could go stay with them. Now my parents have younger children than me and are obviously preoccupied so I could not make them leave their younger children. I am an older brother and she is my younger sister. <br><br>So I went to their house and found my brother in law home. He said my sister was in the bedroom resting and I said okay thanks. I set up my stuff in the living room and then I turned around to see my brother in law walking out of the bedroom with a naked woman and her in his arms also naked. <br><br>I was honestly in shock and my brain just shut down for a second. I did not know what to do. I was so confused and more than anything, I was worried about my sister. I finally managed to say to the man "where is she?" And he said "she is in bed, why?" <br><br>I said, "My sister, where is she?" And he said she was in the bed. I was so angry and I do not even know why. I was angry that my sister was in the bed and that this woman was in her house. So I kind of snapped at him like "is she okay?" <br><br>And he said "yes, she is fine and she knows Krystal is here". <br><br>So I asked "who is Krystal?" And he said "my girlfriend". <br><br>I lost it. I started yelling at him "your girlfriend? You are married!" I just could not believe this was happening. I was so angry and I did not know what to do. I kind of snapped at Krystal and told her to leave. <br><br>He said "sorry bro, but I am going to ask you to leave instead". <br><br>So I left and went to a hotel. I called my parents and told them what happened and suggested they come to check on my sister. <br><br>A couple of hours later my parents called me and told me that when they got there, my sister and brother in law were in the living room like nothing had happened. My sister said "oh, I am so bad with remembering people's names, but she is his coworker". <br><br>It was honestly the most ridiculous thing I have ever heard. I asked my parents if my sister was okay and they said she looked fine but that they were a bit worried about her. <br><br>My parents told my brother in law and my sister that I had gone back home and that they were also going back home. <br><br>I am so angry and I do not even know if I should tell them I saw him cheating. I am just so angry and I am worried about my sister.<br><br>I do not even know what to do and I am so confused.<br><br>So what do I do? <br><br>Please help me, Reddit.<br><br>EDIT: I am back home now and my parents and I are going to have a family meeting about this. My sister does not know I told my parents.