mom thinks i should be the bigger person!

so my mother thinks i should be the bigger person and pretend to like my cousins daughter (whom my aunt takes care of all week) my aunt is always over our house & obviously has the little girl with her; who is 5, my son is 15 months. every time she comes over i have to be watching her & what she does to my son, when she thinks no one is watching she’ll throw his toys at him & she’ll take him from his hands. she doesn’t like sharing & when she brings her toys she doesn’t want my son even looking at them. i don’t mind her playing with his toys but i will not tolerate her hurting my son with his own toys. my mom wants me to act like nothing happens because she doesn’t want my aunt to feel uncomfortable but i can’t help to feel a certain way about this little girl. i need some advice to avoid a discussion with my mom every time they leave