<human>: Summarize the given text in a few sentences.

Throwaway because both of the people in this post use Reddit.

My ex (let's call him Mark) and I have been friends for about 6 years now. We started out as close friends, dated each other for a year, and we're still close despite everything that's happened, so there are no problems between us.

His younger brother (who we'll call Alex), however, has never taken any liking to me, especially when we first met. I tried being nice to him but that didn't seem to work, so I just gave up. Whenever I went to their house I'd remind myself that I wasn't there to hang out with him.

I have no idea what's caused this, but these past few months Alex has completely changed. He stopped making fun of me and insulting me, which progressed into him getting uncomfortably close to me while I was at their house and Mark was in a different room. A few days ago he started sending me texts saying things like, "mark isn't here, want me to show you a fun time? ;)"

I haven't told Mark yet but I plan to very soon. I want to tell him when his brother isn't home so they don't end up doing anything physical, and their relationship already isn't so great. I haven't told their parents either, because they favor Alex over Mark and they'd probably just side with Alex on this. I don't know how to solve this without being blamed for potentially "breaking" their family.
<bot>: ex's little brother is getting too close for comfort and I don't know how to tell him without everything getting horribly violent.