In this case where you're just letting your emotions out on FB. I personally think his family shouldn't get involve or think they have the right mind to criticize over what you say, tt wasn't like you curse infront of his parents or his relatives. This is FB, you say things concerning your mood and how you felt that day, and sometimes you don't really mean what you say. Like they say "Chim chim es cia li hais tau lawm xwb."
I also think that the sister was too quick to act on this situation, which wasn't necessary. She saw your rant but did she even know what her brother did to make you angry? She shouldn't had been so eager to throw the fault on you and point fingers like that. If it was just a nice "You shouldn't say these things, people might think wrong about my brother," I would had been fine with it but when she say you were disrespecting him and ruin his family's reputation she's threatening/warning you, taking it too far.
As for if you should say anything. This isn't something to hype up about, it's just going to make her happy that you got angry. Just stay cool about it. Hmong people can't keep their nose out of things that aren't their problems.