Fiances children....?

He has three kids one of which is his biological child the other two have only known him as their dad. their mom hates me i don't know why. She is the reason for the divorce she cheated on him and wiped out their account. his children mean the world to him she has made it a point to tell him that if he marries me than she will take the kids and will not let him ever see them. it causes problems between him and i because i don't want him to lose his kids. but if it came down to it i would rather him lose me than his children. he gets mad when i tell him that. She makes nasty remarks at me and makes it a point to talk about me to her kids. i treat her kids the same as i treat my daughter when their dad works we have "girls day out" they love spending time with me but they go home and when they come back their mom has told them mean and vicious things about me. If she doesn't want them around me than why drop them off and haul a$$? she says she doesn't want me to be their step mom....cont