Is it normal for my girlfriend to do this?

I am a girl of 22 yrs and living with my mum. Longtime back when I was talking with my girlfriend about her kinks and fetishes she told me that she is into old women also of 40's to 60's. The other day she told me a Truth that shook me, she told me that she is into my mum. I was not able to move or think for a while but she told me that she wouldn't act on this. But after 2-3 yrs she regularly visited my home to make out with me. But after sometime I saw my mum and gf passing smiles to each other which remind me of her fetish. From then on I am not able trust on her or feel good for her. Even though we make out everyday but every time I see both of them sitting very close I can't have good time. If she had did it she should tell me first and I wouldn't be angry on her. What do you think if she had done it is right or wrong. I don't know what to do if she had really done it. I feel like if she had done it I would not stop them from doing so.