My Girlfriend's Mother Is Pregnant For Me; But It's Not My Fault

Dear reader, My name is Elvis, pls I you to look at this situation that I've found myself and tell me if I'm wrong:

 I got a house for my girlfriend out of my parent's house just because I wanted us to have our privacy before we get married. I am always going there to spend the night most times and she did not tell me that her mother is very young and beautiful; a single woman.
But one day I went to "our house" and she introduced me to her mum, the woman really liked me. But I didnt know that it's not just like, but love. One day my girl was on night shift in the hospital where she works and her mum called me around 7:30 that I should come to the house that she wants to see before she goes back.
I went and she told me the house is bored, that I should spend the night with her and stay till my girl is back in the morning so we can both take her to the park to return to Asaba.
I agreed and in the night she woke me up from the chair and asked me to join in her bed...
My heart skipped but she said no problem she just don't want me to have neck pain by sleeping on the chair till morning. I followed her to the bed and in less than five minutes she has taken off her wrapper and was with me under the blanket. I understand what she wanted and by looking at her bare body I was already in the mood, so I slept with her, like 3 times before morning and I even enjoyed her more than her daughter.
When she left, my girlfriend was angry that I spend the night in "our house" with her mum without telling her but I told her it was her mum's idea and that I did not complain since her mum told me that I would stay till she got back from work. I lied to her that nothing happened, that's my mistake.
The mum called me after last Christmas and told me that she was pregnant for me and that we should tell my girl so that her daughter can "cry small and then look for another man". The woman wants to marry me.
My girlfriend is aware now and she is cursing me, threatening my life. But is it my fault? Pls I'm confused!