In my case, I am annoyed because DSD's mother is a charity case to my DSD. In a normal situation, I would gladly take her to buy a gift for her mom and her dad. She regularly informs us that her mother has no money and is broke. Her mother, who gave DH custody last summer, after my husband filed a motion for a legal change in custody so that we could relocate to Georgia from Michigan (although DH has had custody for all but 2 weeks since there divorce more than 3 years ago). The mother does not support the daughter financially at all, despite a prior agreement with DH. She just doesn't know how to be a parent or a responsible person. I get hung up on this issue, and I resent spending money on the mother. It takes a lot of sacrifices on everyone's part to raise this child. I gladly do for DSD but it is not my responsibility to do for the mother.
I also get annoyed because DSD wants to buy a gift for her mother only, but not for anyone else including her father or me. Now, her mother is not rich, but she does work full-time and has few expenses. She chooses to spend her money on drugs and alcohol, rather than her child. I feel used in some way, and that's what bothers me. I try not to let this resentment cloud my perspective, but I am only human.