I have a brother and a sister.
My sister is 10 years younger than me. We are very close, she is very close to my DD's and I adore my niece and nephew.
We live a few hours apart and see each other every other month or so. We talk almost every day. I get on very well with BIL as well.
My brother is 18 months younger than me and I have had no contact for many years. He was aggressive, abusive, drug addicted gambler, and after numerous chances, and some very odd comments (apparantly I sexually abused him and helped our father abuse him, we were both children in a crappy family with an abusive father and an absent mother - she had left with my younger sister, I always felt I tried to protect him, so accusations like that were unforgiveable). When his life is good, he is effusive in his charm and spending money etc etc, when bad he is horrid.
After being physically abusive to my sis and BIL, she had no contact with him, but has recently built bridges, and is seeing a bit of him, despite BIL reservations.
So I get to see all their "fun" escapades on FB (bloody FB), "lunch with my gorgeous sister", "babysitting the best niece and nephew in the world", "has the most gorgeous niece" etc etc. I can see them because he tags her in them all. And I am consumed with jealousy, he lives much closer, so gets to see them much more than I do. She seems to be having a ball with him (he is taking her to fancy expensive restaurants). He gets to see my lovely niece and nephew much more, but clearly doesnt care about his other nieces.
I want nothing to do with him, some of what he said was unforgiveable, and I have been here before. He will be fine, until he isnt, and then everyone involved will get hurt. So on top of being jealous, I am worried about my lovely family. And yes IABU I know that, I am a grown up, as are they, I need to get over it.