"My Sister in Law is Flirty on Facebook"

Dear Cybersorter,
My brother’s wife is very flirtatious with men on Facebook. She will often write a comment whenever they post even the most banal rubbish about their lunchtime sandwich.
She’s always been, errm, bubbly, but now she’s bordering on risqué. Recently she posted a You Tube video of Marilyn Monroe singing Happy Birthday, Mr President on her male friend’s wall for his birthday. I think she’s going too far.
I was going to tell my brother about it but they are friends on Facebook so he must know. How can I get her to stop?
AP

Dear AP,
From your email I get the impression you don’t much like your sister-in-law. You don’t exactly call her a selfish harpy but it’s there between the lines.
If your brother hasn’t mentioned it then it’s probably best not to bring it up. It might pain him to discuss something he has chosen to ignore. This doesn’t excuse her neurotic and narcissistic approach to social media relationships.
It’s not cool, when married, to stalk around Facebook waiting for cute guys and hanging on every word they post. For starters, it grossly swells the subject’s ego. Something we could all do without. I suggest one of two options:
1)Every time you see her flirt, post your own comment underneath: “Oh sis-in-law, you are SUCH a flirt ;-)” or “Sis-in-law, how can you write that when you know cheese sandwiches constipate you!?”
It is bitchy but it lets her know you are watching her behaviour. Hopefully, this will embarrass her into chilling out with the hot type.
Set a time limit on it. You don’t want to stalk her or ruin her online life. Well, maybe you do, but I don’t.
2)De-friend her. It’s your brother’s private life, not yours and it’s essentially harmless as far as you know. You may be the only person bothered by her behaviour. So stop looking at it.