AITA for telling my step son's mom no about a Christmas present she bought him?

I am 41 and my husband is 43. I have a 14 year old step son who is currently in therapy for his anger issues. He's a very.... Disturbed child. He has put a lot of holes in my walls and gotten into fights at school. I know he didn't have a good home life living with his mom and step dad. I love this kid more than anything and want him to get the help he needs. He has lived with us full time for a year because of issues with his mom and step father. He saw his mom for Christmas and she gifted him a knife for Christmas. It is illegal in our state for anyone under 18 to have a knife and as I've said. He's a disturbed child with really bad anger issues and when kids at school make him mad he resorts to fist before he resorts to words. I don't think he would harm us but definitely someone else if he was angry. I told my husband I didn't feel comfortable with him owning a knife. After talking we both decided he didn't need a knife so he told my step sons mom to come get the knife. She was upset about this and said she bought it and she's the mom. How I see it is it's our house. I don't want him having a knife in my house. It's not legal for him to have one anyway. She believes we are undermining her parenting but we live with him. I feel like I should have a say of what happens in my home. But she thinks I'm undermining and being a bitch

@Melissa I literally said "He told my step sons mom to come get the knife."