Bit of background: I am the oldest of four children and have a younger sister who lives close by. We both live near to my parents while our brothers are further away. I have two girls aged 2 and 4, my sister has a 1 year old girl.
My mum looks after my youngest one day a week in term time (I am a teacher) while my sister takes her child to work with her (she is a nanny). Because mum mum regularly babysits so I can work, we hardly ever ask her to in the evening. As a result my husband and I almost never go out. I accept that I chose to have the children so not blaming anyone. We maybe go out together without the children three times a year. My sister goes out pretty much every weekend and my mum (or her mother-in-law) babysits.
Anyway, we asked my mum if we could go to the cinema this Friday since we can't even remember when we last went - and in fact she offered to take the children so we could go. Today I popped round and she mentioned that since agreeing with me that she would babysit, my sister had asked her to have her daughter so that she could go out. Therefore she said that we could only go to a matinee so that she could look after my niece in the evening.
AIBU to be super pissed off? This is not the first time it has happened. I feel my sister's needs / child is prioritised over mine regularly.