Future MIL doesn't want my husband taking my last name

My future mil has a belief that when a man and women get married a women marries into the man's family. Not the other way around. She believes that taking a man last name is a wife saying she is is and showing loyalty to her husband. I think loyalty would be me being faithful. But anyway he's the one deciding to take my last name. His last name is Spooner and mine is Karrington. She believes my last name is too feminine anyway. But my fiancè hates his last name. His last name is the last name of his mom's late husband who my fiancè was raised to believe was his dad but he actually met his mom while she was pregnant with him and signed his birth certificate. My fiancè was sexually abused by this man from 7 to 19. This man also gave my fiancè herpes and gave him syphilis when he was 15. His mom walked in on the abuse several times but never did anything. My fiancè stopped talking to his mom when he found out the man wasn't his real father anyway and he found his real dad. My fiancè's step father died 3 years ago and him and his mom have rebuilt their relationship. She's seeing him take my last name as me stealing her son from her and said I'm manipulating him. My mom doesn't agree with her reasoning but does agree it's odd for a man to take a women's last name but told me it's our life and do what we want. His mom said if he takes me last name she will consider it a betrayal and him saying he no longer wants to be a apart of the family. My future sibling in laws think we should just give his mom what she wants to keep the peace. She believes he's leaving the family but his step dad isn't even family.

@idontcare she's already aware of the reason he wants to change it. She believes since her husband is dead the trauma should be too.