I don't think so - I've done that in the past with my father in law as it seemed like everytime DH or myself mentioned anything about having a drink or alcohol he'd get up on his soapbox... one time he even went off on DH for mentioning drinking on 4th of July in his status, claiming people on his list could see it, etc, not seeming to understand how privacy settings work and was refusing to listen when we tried to explain that no, we have our profiles set to private, unless his friends are our friends (and they aren't, except for an honorary 'uncle' of DH's) they cannot see what we post. it got quite annoying so we both set our profiles to limited view when it came to FIL and honorary uncle). And what we posted was pretty tame compared to what our 'friends' who are still in college post on a weekly basis, lol...
Sibling dating drama ALWAYS sucks - especially when you know and have connections to both parties. the best you can do is remain neutral. My 16 year old sis and DH's 17 year old brother dated for like a year after our wedding despite us all telling them it wasn't a good idea and they just broke up last fall. Let me say, it HAS NOT been fun for us hearing all the drama the past few months(and little sis being little sis comes to me and mom for advice too) - hearing my brother in law has been saying some mean stuff about my sister, and then hearing that she's said some mean stuff about him the past... it's typical high school but it sucks being the middle as DH and I related to both of them and yet they can't be civil towards each other now... one thing everyone keeps telling us, it will blow over eventually and they'll grow up and get over it someday. So just keep telling yourself that - hopefully things will work out with them and everyone will be able to just get along and be civil at some point.
I tend to write long posts - short and simple doesn't exist with me! That's how I am, take it or leave it.