Hi I really need some advice please

I was with my ex for 8 years and we have 2 children together. We went through a bad patch and split up. We both got hurt from it very bad from splitting. This was a year ago now. My ex vented to her cousin who started interfering in our relationship. Now this did not go well with me because it was nothing to do with her. Naturally enough I told her to keep her nose out because it was none of her business. This girl loves drama and has to be in the middle of it everytime. She ended up coming between us and told a lot of lies about me to my ex who believed her at the time. My ex ended up depressed over all this and needed space. So I gave it to her. I moved into my mothers and her cousin ended up staying in my house. As time went on my ex and I started talking and 1 thing led to another and we vecame intimate again. When her cousin found out she told my ex she will kill herself if she gets back with me. so my ex decided she couldn't because of this. Now I know it's lies but my ex believes her. Anyway my ex is still fragile so I backed off. We hit it off again soon after again and the cousin reads her messages and found out, she hit my ex and left her black and blue. At this stage I got angry and threw the cousin out of the house but my ex felt sorry for her and let her back in. The cousin took her phone and any time I txted she wud delete the messages and not tell my ex. This girl is ruining my life at this stage. My ex and I hav been split over a year but still she is the only one for me and I am for her but she is afraid if we get together that her cousin will go off the rails. This girl is on drugs, has dealers chasing her for money. She doesn't care about my ex just wants to control what is going on. My ex told me she loves me and wants to be with me but she is afraid of what her cousin will do. I can't even go into my own house to see my girls unless she is not there and she fights with my ex if she knows i called up. My ex wants to be with me and everytime we get close I have this cousin causing trouble. I am not the type of person to allow someone get involved into my business but I can't do anything because I don't want to push my ex away further. It is ruining my life I can't think of anything else and I don't know how to deal with it. I just want my woman and children back and the more time that passes the less likely it will happen please give me some advice. My ex's family told her to get her cousin out or she will ruin her life. This girl lived with her nanny but hurt her with a weapon because she would not give her any money one day and that is how she ended up in my house. Please read this and give me your opinion

Thank you