It's not about whether you ask her to. If someone gives a gift, they don't want to hear "You didn't have to." They KNOW they didn't have to. She wants to be needed and appreciated, but doesn't trust that she is, so she starts the "Oy, how I've suffered" routine. Which I agree is obnoxious and more than you want to hear. It may also be that she likes the "thrill of the chase" meaning the act of shopping, but she doesn't get the same "high" off the actual giving.
I'd meet her halfway. Make sure that your children are writing really awesome and heartfelt thank you notes, nothing perfunctory. If one is too little, then YOU write the note as if it's from that child. Otherwise, it's a really important skill to develop so sit with the older kids and either write what they dictate (even if it's childish!) or enhance it a bit. That's from 4 years old on. One they are 7, they write their entire not with you helping with an "outline" (mention the gift by name, how the child has already played with it or what they plan to do with it, what their siblings or friends said about it, and finally what they think of an aunty who takes the time to choose a gift. Make sure it's not about the money but about the time and thought.) My son was raised that he could not play with the gift on the 2nd day (only the day when he opened it) or spend the cash/cash the check until the thank you was written. If he didn't want to write the thank you, I told him he could write a letter to the gift-giver saying why he didn't appreciate it and why he was sending it back. So either way, he was writing a note!
I can tell you that I am still sitting her waiting for promised thank you notes from a relative's family for 2 years - the dad does nothing, and the mom keeps saying, "Susie has her note cards and stamps, and she'll be writing her thank you notes next weekend." Nothing for the past 2 years of birthdays and holidays. So I'm done giving gifts.
Have the kids MAKE gifts for her that take their time, not your cash. That could be cookies, drawings, crafts, ANYTHING that shows they care about her and not that you went out and spent money. Supposedly, this aunty is talking about how much she DOES more than what she SPENDS, so you should repay her in time and affection, not money.
If you do the above for the next round of gift-giving (Christmas or whatever it is), and if she STILL persists in ranting about how hard it was or (worse) how much she spent and how little she got in return, then I'd sit her down and tell her that the cost accounting goes out of gift giving right now, and that either you just write her a check and she writes you one, making it exactly even. Tell her she is taking the thrill and joy out of the gifts for the kids, and making them resent her, and is that what she wants. If it is, then she's not going to be allowed to bring gifts for any occasion.