Sounds familiar, MIL turned up with weaning spoons, when I'd been looking forward to taking dd into town to buy weaning stuff, she also appeared with pants just before I was ready to start training dd.
It's annoying because she's had all those little, but important, firsts with three children of her own.
Anyway, each Christmas I try to get my pil and my mother to reduce the number of things they buy and it seems to get better each year, but there's still an element of competition.
One thing that seems to work is getting them to buy more practical things that dd needs anyway, like PJs, slippers, dressing gown, pants, christmas day dres, socks etc so they still feel they've created a bit pile of presents but at least most of it isn't plastic crap which I don't have room for.
She's getting to an age now where she wants smaller but expensive things like Wii games, and there was a danger of them compensating with lots of smaller cheaper things to make it look more, so I reinforce the message in the run up to christmas.
I also squirrel away things to bring out later in the year when dd isn't so over whelmed with stuff (also because it's her birthday just before Christmas so she gets lots of stuff almost in one go) presents get much more attention and appreciation when I pull them out of the back of her cupboard in March.
Sorry that was really long, one of my pet subjects.