<human>: Give me a shortened version of the text

Spending Christmas break at my parent's house, and my sister came over to visit last night, and stayed the night. I was walking into the kitchen this morning and hear my mom and sister talking about my sister's one night stands.

Sister was saying that she feels a little guilty, saying she loves my BiL so much, but she just can't help but have sex with a new guy every now and then. Mom told her don't feel guilty it is natural to want to be with others, just don't get emotionally involved and get caught, and no one will ever know, have a long happy marriage.

This hit me in the gut, especially since my mother went through cheating with my Dad several years back. He had a full on affair with a woman for months, it was a tumultuous time, but Mom forgave him and took him back.

Now I just see her hypocrisy. I have not been around either of them all day and have stayed in my room, because I don't want to be around them.

Is it true everyone cheats, I mean, jeeze, not giving me a whole lot of confidence about any future relationship I will have?

Should I tell my BiL and my Dad what I heard? I have no evidence other than listening to their conversation, and I wonder if it is my place to say anything, but I just feel like they should know. Especially the way my mother was so callus about it. If you truly care for someone, wouldn't there be some guilt? I know she said it is how you have a long happy marriage, but wouldn't it be almost a fake marriage?

I am just lost on what to do.
<bot>: Sister is cheating on BiL, Mother encouraging her, possibly cheating on Dad, unsure of if it is my place to tell Dad and BiL