Daughter doesn't like my boyfriend

Me and my boyfriend have been dating for 8 months. I waited to tell my 15 yrar old daughter until it was more serious. She seemed fine with it at first until she found out he has a 6 year old daughter. She lives with him full time. My daughter has always been the only child. Even when her father remarried he never had anymore kids. I didn't really date for a long time so I never had the option to have more children. She says she doesn't want to meet him or his daughter ever and told me I better not be thinking about marrying him. Did anyone have this problem and did your kids come around?