Ok, so first, a little background: ( I'm not sure how much is relevant but don't want to dripfeed). Sister and I both have children, similar ages. She has two, at school, I have three, one not quite at school age, other 2 at school. I'm married, sister engaged. 5 years between us. Sister has never worked, her DP is in a dead end low paid job, although he occasionally makes noises about changing career he never does. My DH has a trade and works a lot of hours to keep us afloat and to allow us a (low key) holiday every year. We're not well off, but we do Ok. I've recently gone back to work for added financial security (not ideal as would love to be at home with youngest, but it is only part time). I also do DH's books to save accountancy fees. Sister doesn't drive, I do.
So my parents help out my sister, a lot, because of their (self inflicted, IMO) financial postion. To the point that they are unable to help us as much. For instance, my Mum offered to help with childcare for my youngest when I went back to work, but was unable to because of a commiment to taxi services for DSister. Family events are always at our house (Mum's is too small and sister couldn't possibly afford it) and we never get more of a contribution than a bottle of pop and barely a thanks. It's always excused because of their 'position'. Also, if we ever arrange to go out with the children, I have to collect and drop off, either that or the cousins don't see each other.
So all of this, although I can be resentful and frustrated sometimes, I can cope with, but (and this is the AIBU) my 'big one' is coming up and my DM has made a lovely suggestion that us girls should do something. The 'something' comes at a cost of £100 each, which my DM has generously said will be her treat as my present, however as DSis is in 'a difficult financial position' she is going to pay £50 towards hers and £50 towards mine, so that she doesn't miss out. As compensation for this she has said she will do the same on DSis's for me (in 5 years time). I'm surprisingly pissed off about this. Almost like the final straw. AIBU?