Mine is my sil. Yet again there are photos of her on Facebook getting pissed on a Saturday night. She's over forty and has two kids. Meanwhile her recently widowed mum has to look after her kids for her all day today yet again because she is hung over and partner has to work. Dh and I have not had a Saturday evening together since his dad (who is sil's dad) passed away because mil comes to stay with us every weekend. Sil hasn't offered to have her once.
Yet everyone thinks she's wonderful and that her partying is hilarious. I think it's pathetic and selfish and completely gets away with it. That's why she's the most annoying person in my life.