Grandma spoiling

So I have three kids one of which is my current boyfriends. Well his mom likes to buy the baby stuff but never includes my other children and it honestly upsets me that only one kid is getting things because the other ones notice. And I have to explain over and over to them that she isn’t their grandma. Is there something any of you ladies think I can do? I don’t want to be mean but I want to tell her not to buy things for baby if she can’t the other kids.