may be you should try to be a little more nice to her. i have noticed that there are people who feels lonely and sad and they try to make themselves look happy by doing things. like posting happy pictures on social media. may be she is one of them or may be she is an arse, as you said. but she is family, so may be you should try to make a conversation with her. a gentle one.
but everything i am saying its just my opinion. i don't know her as you do. and i also get angry sometime by my few relatives.
If it was a stranger them perhaps I would be nicer. This one however I have known for a long time and know exactly what she is like!
then i hope you are totally right.