Have to be honest, you're situation doesn't sound ideal.
1. You've just broken up with your girl, so you're perhaps using this girl as a rebound.
2. She's obviously not the most faithful girl in the world. What makes you think you're the special guy that will change her?
3. If you are the special guy that's going to change her, why is she still with him?
I'm sorry that these questions come across a bit bluntly, but I think you need to consider them as straightly as possible. I feel for the situation you are in but you need to think about the long term plan.
And, her complaint with her boyfriend is that she's not getting any (sexual) attention, and she's cheated twice. Sounds like she's happy with him but just wants as much sex as she can get. There's another word for that... but I don't know her so I'm not going to start calling folks names.
I broke up with my gf because of this girl, I knew I wanted her more than my Gf so ended it as it wouldn't have been fair to treat her second best.
she hasn't been faithful to this one but the last was three and a half years and she never played around.
her complaint about her bf isn't lack of sex, they went for a meal the other day and he was more worried about a dry bit of skin on his ear, the traffic going past and his phone than her.
I don't think I'm anyone special, just hope that I could be.
My trouble is that no woman takes an interest in me unless they are already in a relationship, I haven't yet had a girlfriend that was single when I met them, they all left someone for me, I was never trying to get any of them, this one though is different, I am just going out my mind for her, I would have walked away by now had it been any other woman.