I really feel for you, I know exactly what you mean!
We have dts. My dp's brother lives with their aunt and gran. His bro and aunt have no children of their own. The three of them are reasonably well off and totally spoil the boys.
Most of their clothes are bought by them, and they don't do returns, so I end up having to use it. Some of the things I really have disliked, such as camoflage styles. They buy shoes that haven't been properly fitted or are unsuitable. Most of it is top designer clothing, that I couldnt afford. Often it is really lovely, though I couldnt jsutify the price tags.
They buy massive toys that we don't have the space for. (we have a three bed house, and the three boys share a room). Often they are toys that are too old for them and jsut end up broken. They bought a new car seat last week because ours was squeaking. They bought blankets, their first weaning set. A wooden playhouse for the garden, their first trikes.
They went to India in the spring, brought back 7 indian suits each for them, which they still havent worn one of. (They wanted to take the boys with them, but I put my foot down).
They paid for us all to go to florida for a fortnight, so they could take the boys to see mickey mouse. They took hundreds of dollars to spend on clothes and toys for them, they bought pretty much anything they pointed at. They took them on the first trip to the cinema, and their first time bowling. They even offered to pay for preschooling.
I do feel that they are too involved and take over too much and I resent it when they take things away from us that we should be doing, especially 'firsts'.
Talking to them gets us nowhere. We once sold some new bits on ebay and they werent happy, but it didnt stop them neither!
I really have given up now, and I am grateful for a lot of what they do, so I have to take the bad with the good. It means the boys do get things and experiences that we could never afford. I do try to get dp to make suggestions now, to try to influence what they do buy, but it is mostly out of my control. Some stuff just ends up in the back of cupboards until it's outgrown and then it ends up on ebay.
ours rarely do the babysitting any more neither, not once they got to toddler and answering back stage. They just want the fun parts.
There jsut isn't really anything you can do in this kind of situation. You can't confront somebody about buying too many presents without totally pissing them off! I do sympathise with you totally. About the spoiling though, they do tend to learn taht it is coming from one direction. Mine tend to ask auntie when they want something new! They know I won't buy everything they set their eyes on. They do play up more around them, because they do spoil them, but away from them they are generally ok and don't expect it from us. As long as you reinforce your values in your home, your dd will be fine.