Hi Simone,
Thats a hard one. I dont think you are being unreasonable, but she obviously really wants to do this for them. maybe try to come to some sort of compromise. When our parents ask what the girls want, we try to give them a list of things they need, so if you gave hem a list, maybe put the less expensive things on there. ie, this Christams, we've said the girls have enough toys, so my Niamh is getting a duvet set from my mother in law. She'll probably get a few little things aswell, but its nice for the family to feel like they are getting useful things. We've found lists work really well, as nobody gets offended then.
Hope this helps a little bit.
Kas xx