My Girlfriend Cheated On Me With My Twin Brother

UNEDITED:

I met this girl 3 years ago when she was posted to the office where I work. So she was introduced to the various staff and myself inclusive. From time to time, I used to check on her to know how she is catching up wit work.
One tin lead to another and we started dating. At that point, I introduced her to my twin brother thru Facebook cos I hardly see him due to d fact dat his office is very far from mine.
My relationship wit her was goin on smoothly. And sometimes, she tells me dat she gisted wit my twin. I didn't really tink dat anytin could happen between them.
After about 8 months of our relationship, we had a little misunderstanding and were trying to resolve it, but she always turned down any effort I made towards resolving d problem failed as she was always raining insults on me. Meanwhile, d issues causing d problem wasn't my fault.
There was a day I visited my twin brother and while I was there, she called him and told him she was coming. When he told her I was around, she said she can't come. At dat point, I tot maybe she was reporting me to him or she was telling him she was still angry wit me.
From then till the next one year, I didn't see her and didn't hear from her again, and my brother didn't mentioned her to me either.
So a year passed and I decided to call her with d intention of reconciling wit her and proposing marriage to her. So we started getting along well again as before although it was thru fone calls.
So I told her I wanted us to see, I had d intention of proposing to her. So she told me dat she would come.
Later dat day, she called me and was crying, and said there is something she must tell me cos she didn't want me to hear from som1 else. I asked her what it was. And she said while we had our misunderstanding, she was sleeping wit my twin brother.
From d momement she told me she was having sexxxx wit my twin brother, every tin I felt for her died immediately and I told her dat there is no point of us even talking anymore

But I couldn't ask my twin brother cos we r not close to each other, we have never attended d sameschool, neither primary, secondary nor university. So our relationship is near formal.
Secondly, its possible she is lieing about sleeping wit him, so if I aske him, he wil see it as a false accusation.
So I just decided not to ask him.
But the girl has been begging me to forgive her, so I went to seek advice from my boss, he told me to forgive her and not to drive her away from myself. I told him I wil try my best, but dat I cannot date her.
Now d problem is dat she is assuming dat we r dating again. And is asking me why am not allowing her come to my house or why I don't go to her house if I say I have fully forgiven her.
I told her av forgiven her but I can't date or marry her.
She keeps on begging me, now I really don't know how to get her off my back.
Dear Readers, pls what do I do to get her off my back, cos I can't feel anytin for her. And secondly how do I ask my twin brother so as not to cause a problem between him and I?