How to break up with a girlfriend you love?

So I have an issue I have been going out with my girlfriend for almost 6 months it is starting to get serious. I was talking to my mom and we talked she asked the basics and I told her that my girlfriend has bi polar disorder. She tells me all these horrible things about people she heard of who has it and she tells everyone else in my family about it so my grandfather and my dad. My mom and grandfather are saying break up with her or we will disown you (didn't actually say it but thats what they meant and my dad is not saying anything. I know I'm an adult 21 can make my own choice but this is my family so I am going to choose them. I need ideas how to do this so it doesn't turn ugly or end it on a good note.