sorry this is going to be a bit long but you need the background really. My sister is my Dad's daughter from his first marriage and I have always had a bit of a difficult relationship with her. She is 18 years older than me (my mother was my Dad's third wife and sixteen years younger than him, so there wasn't a huge amount of age difference between my sister and my mum) and when I was little we were quite close, but when I got older and started having my own opinions I realise she was quite difficult. It's incredibly easy to offend her - I won't go into the list of what does now as I would be writing forever - and she has got the most enormous chip on her shoulder and a completely false view that my Dad 'favours' his children with my mum (there are three of us) over her.
This is untrue for a couple of reasons: My Dad is an appalling, absent father. My parents split up when I was fourteen (he ran off with one of her friends) and since then my mum has largely done the parenting alone (my other siblings are all younger than me, I am 25). He treats us all the exact same way her treats my sister. The only reason she had more experience of him being absent than I did is because her parents split up when she was two or three. My brother, incidentally, was only two when my Dad left my Mum so he has had the exact same experience, but she has this major victim mentality that insists she was the one who was hard done by.
She had a baby a couple of years ago and that improved our relationship a little as she seemed to soften somewhat (she is usually very hard, unaffectionate and can be quite bitchy and spiteful, particularly towards my mum who I suspect she is a bit jealous of). I love my nephew to bits. She then split up with the baby's father which was not a surprise as she is really an incredibly difficult person to get on with, as I have said. So she then moved away to another county, in the middle of nowhere, with no car and no nearby train station. So it is really difficult for us to see her - my mum works six days a week including nights and my siblings and I have no viable means of transportation.
Anyway, my mum invited her for christmas this year, as she always does, and my sister ruined the whole occasion: she was rude to us, rude to our other guests, didn't contribute anything, moaned about the food, sulked in the living room away from everyone else for most of the day and wouldn't let anyone else near my nephew.
Three days later I realise she had 'unfriended' me from facebook, which really upset me as I have got literally no idea why. She hasn't done the same thing to any of my other siblings, or to my father (who she always claims to be furious with). Why she has singled me out I have literally no idea.
Any advice on how to deal with this at all and AIBU to get so upset by it? I have been sitting on it for a while and don't know what to do. Happy to answer any more questions if needed.