His niece was/is my ex boyfriends side piece... needless to say. LONG story

SORRY THIS IS SO LONG I DIDNT WANT TO LEAVE ANY GAPS TO ANSWER REPEATEDLY!

I'm dating a wonderful man named Cole who I've known for years, even though I'm 19 and he's going 21 in a few months, I can barely picture anything coming between us. But there's always something 😧. It just so happens that his 16 or 17 year old niece is the same person that my ex cheated on me with. I found this out August 2014 when I went to his house to ask him if he still wanted to date me. ( my ex and I broke up January 2014, my now boyfriend and I went movies towards the end of January and he told me he really likes me and wanted to date me. At that time I was still hurt and told him no.) But when I went into detail about what happened with my ex he told me the girl is his niece. I backed off.

I had thought that if I got with him he'd think I was only with him as a instrument to hurt his niece, instead when I started up the college I started to like a different boy. For months I liked this boy until I realized I liked it for all the wrong reasons. ( he looked almost EXCATLY like my ex)

In April 2015. My current boyfriend and I went movies again and I had one of the best nights of my life with him. We had so much fun and I decided if my intentions are true and I'm not using him I should be with him. After all, I didn't know that was his niece until he told me, and he knew his neice was with my ex when he asked me out.

For a while ever thing was perfect until around the end of April one of her friends accused me of only being with my him to spite her. I told him about it, he told me not to worry about her. In may she came down to his mother's house. When we came to the house she was there. He asked me not to hit her (which I wasn't even going to do) and we went in.

She was trying to start something but I guess in the end hurt herself, I acted as I would normally, as I had been doing for years. I played with who I now know to be her brother, and talked to who I now know is her father. After seeing this she left. But it wasn't over. About 2 days later she accused my boyfriend of deleting her off facebook. (She messaged Che which is Coles twin bro) Obviously he told her it wasn't him. She blamed me saying it was your girlfriend. He just ignored it and added her back. She tells him that she isn't speaking to cole for dating me or che for being my friend. In June she tried some other stuff but nothing serious. Whenever she starts a rumor it stays within her friends or she writes something on her Facebook that is suggested towards me. ( I know because one of our mutual friends tagged me in a few statuses of hers but on of them delete me after I receive the notification)

Just in case anyone is wondering, I'm not sure if her and my ex are still together. In November 2014 we got back together and apperently he was still with her. In March I decided I don't deserve to be treated like that and left him. (Best decision EVER)

so my question is how would you handle his niece if you were me. He's already said he doesn't care for her childish games but I think she's going a bit far. I haven't heard anything lately but that doesn't mean nothings going on right?