Relationships

How can I make my boyfriend's brother stop sabotaging our relationship?

I've been dating my boyfriend over a year and a half now. We moved to a new city and live together, I'm attending uni here and he's working. He has 4 brothers, the eldest was really nice to me first but now, he's so mean to me for some reason and I honestly have no idea why.
He wanted us to spend new years eve with him and his girlfriend at their home, but we had a plan we made for nye two months beforehand so I told my boyfriend that there was no chance that I'm spending that day with them, I offered him however that he can stay and celebrate with them if he wanted. When I was about to leave, my boyfriend made me really angry and I told him to shut up because he was being really annoying, to this his brother started yelling at me telling me that I was acting like a 12 yo and I have no wright to talk to my boyfriend like this.
Yesterday when I opened the laptop, my boyfriends facebook messages were open, one from his brother telling him, that I don't have it public on my facebook that we're dating (I do have it though, but I'm trying to keep my family protected and myself, I never was the oversharing type of person) and I quote "this chick has been making some really weird stuff, I think it's time for you to move on". I told my boyfriend that I read the message because it was open, and that I wanted him to stand up for me and tell him to mind his business. Another thing I would like to add, that his brother's girlfriend is expecting their first child. So why the hell is he trying to sabotage our relationship? What am I supposed to do? I don't want to message his brother about it. It just makes me feel really really bad, because I'm trying so hard to have a good relationship with his family, and they're always talking behind my back. I'm not used to this, my family never would do this to him. Any opinions? Help please?
19/Fe
How can I make my boyfriend's brother stop sabotaging our relationship?
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