<human>: OK so last night that happened. We've been together for a year.

We were visiting his parent's house (who live by the beach) and his brother was also there with his wife. We stayed there last night and at around 3am I was half sleep and noticed that he's leaving the room. I thought he's probably going to the bathroom or something. I heard some noises from downstairs and I wasn't paying attention until I saw from the window that he's going out to the beach with his brother's wife!

They came back about an hour later.

I still haven't confronted him and was wondering what this could mean. What was it that they needed to do? I don't know what to think. Is it something that I can be upset about? Sneaking around at night isn't cool in my opinion.

Condense the text into a few key points
<bot>: Boyfriend sneaked around at 3am with his sister in law to go to the beach and came an hour later while he thought I was sleeping.