Boyfriends brother flirting with me?

Anonymous
This started kind of recently, at least from what I've noticed. We had a game night and his brother told me " I should smile more and that I had a beautiful smile". My boyfriend was also pissed that night for another reason. later on that night, he asked and I quote "Why are you with him?" he said it lowly but I pretended not to hear it and asked him a different version of the question, like I was clarifying what he said.
He's always happy to see me though and once their sister told him " you better stop talking to his girlfriend before he gets mad"

Tonight I shared something on Facebook saying "men only, give women dating advice" he commented and said, "let a real man love you". I said "welp, you don't have to tell me twice" (other guys commented as well). he literally replied saying "HMU and left his number"

I don't know what to do. I have the other brother and sister and law as friends. They might see it, as well as my boyfriend if he goes on Facebook.
Boyfriends brother flirting with me?
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