Am I being unreasonable here?

My mother wants to buy my 3 yr old a power wheels for Christmas.We live in a small upstairs apartment.I dont have room for it and can't carry it and my kid downstairs, so she could only play with it when her dad is home.We're moving to a house in 3 months my daughters birthday is in 5 months,so I suggested she get it then.She said ok but I found out today she is really mad. She umed my husband told me to ask her not to get this to hurt her.(I really don't know why she thought that)Today she accidentally sent me an e-mail meant for another family member about what she was getting my daughter, she says in the letter ".....and a drum set. (son in law p**sed me off again). Kinda hard for him to b**ch when I'm just spoiling the grand kid because she loves music. Tee hee He doesnt even know he p**sed me off. I love it. The mind f**k can be so sweet. haha" This is just the latest in a long history of stuff like this and I'm ready to just cut her out.What should I do?