I need a bit of perspective but please don't be too harsh as I'm quite upset.
Bit of background - I lost my mum in my 20s and my dad remarried a while back. I really like my step mum, she is fab. I'm happy my dad is happy.
My step mum has always spoiled (IMO) her kids. As a result my step sister is IMO spoilt, entitled and often inconsiderate. We can get on OK but she winds me up soooo much. Basically, rude and unappreciative of her mum who gives her a huge amount of help. Ungrateful and rude when I've offered her any of my kids stuff (I've stopped doing it now), moans a lot including most of Christmas Day when we'd cooked and ran around after her and her kids all day (no thank you until a text nearly 2 months later!) She has got her hands full with her kids and her husband away a lot but she doesn't seem to appreciate what she's got (i.e. a lovely mum and fab brother, not to mention amazing step dad!) She never really asks about me and when we had a couple of bereavements there was complete radio silence.
The worst thing about this is that she goes to her mum's (my old family home) every Sunday with her kids for lunch. We are rarely invited and if we are I don't really enjoy it as she winds me up by basically sitting on her bum and letting her mum and my dad run around after her whilst she moans! I get really upset because, whilst there is no way we'd want to go every week, it would be lovely to go there without her and the kids being there.
I have spoken to my dad about it but I hate upsetting him. After the last chat about it (basically I just asked if we could come occasionally without them, even twice a year would be enough!), we got asked last summer to a BBQ, just us, and it was lovely. But nothing since. It's hard for him as my step mum, whilst lovely, can be difficult around these things and go in massive huffs if her kids are left out, making my dad's life v difficult. 
I've just spent time with them all and I honestly feel exhausted by it all. I know there is an element of jealousy as I wish I had my mum but I'm generally a positive person who doesn't dwell on things but this is eating away at me. I really can't stand this woman sometimes and it's not a nice feeling. It's affecting my relationship with my dad which makes me sad as we are very close. I don't want to look back and regret my behaviour when he's no longer here 
So, mammoth epistle. AIBU and if I am, what can I do? And if IANBU what can I do?!