Does Anyone See Where I'm Coming From???

Me and my boyfriend just had a big fight over our kids. His daughter is 19 and she lives with us. I have two sons. One is 15, one is 6. So his daughter and my 15 year were in the living room, and I guess he was being annoying. He was playing his YouTube really Loud and when she told him to turn it down he said something like "turn your face down". Like normal little brother stuff. So she gets up and leaves then comes back saying, "look what I've got". It was a poo bear my husband had given my son when he was 7 for his birthday back when he was really into stuffed animals. My husband died before my youngest was born from cancer. Everyone knows why he keeps and sleeps with his poo bear. She starts cutting it with scissors. I hear yelling down stairs and see the commotion. My son is angry crying and looks like he might punch a wall so I sit him down and tell him to calm down. I ask what happens and he tells me. Then my boyfriend's daughter said "Well he was annoying me!" I told my bf what happened ans he just shrugged and said "He's too old for it anyone. Its not gonna bring his dad back". I can't believe he's being so insensitive. I feel like I can't say anything to her because she's not my daughter. So all I can do is try my best to sew it up. I feel she went way too far.

Update: I was able to save the bear! I may not be great at sewing but I did a decent job. My son left over his friends house with his poo bear because he's still upset. Thank you guys for seeing my side. I'm just so annoyed. My bf expects me to punish my kids every time they do something wrong which I do, but if his daughter does something wrong it's never a problem. Even though she's much older than my kids.