ok, lots of back story but in a nutshell my big bro is given every free ride going by both my (separated) parents. To the point that he now even has an interest free mortgage provided by one of our parents despite him being perfectly capable of paying his own but choosing not to; one o those fully fledged adults that flops from crisis to crisis and expects t be bailed out as life constantly surprises him. He has a good job, and a wife and child and should be perfectly capable of having a very stable life well within his means. Instead he will still suddenly call up to say to car insurance is due etc (because obviously this can't be predicted) and he doesn't have X amount to pay it. I have suggested he could direct debit it monthly etc but he says no as that would cost more so gets one of our parents to 'loan' the money but never repays it.
Anyway, long story short our mum has a very significant birthday soon, I have saved up a pretty significant sum of money to order her some jewellery which has been on order since nov to track down specific stones and get made. He asked today what our plan were and if he could get in on it. He has offered to pay less than 10% of the cost but want to go in on it with a card etc. AIBU to feel really bitter and want to find a way to say no (which seems really mean as I know she loves this item of jewellery) but I have gone without to save all this money and if it comes from both of us surely she will assume it is 50/50.
I feel like i am being cruel to my sibling as I can't suggest anything that i know she would like as much as I know she would like this, but I have planned and budgeted for this for almost a year and he now wants in 3 weeks before.