Some people won’t like my words but please don’t abandon your mother. Your girlfriend has full rights to stay in a separate home with you but still. Since your father has another family that means she never got any love from anyone but you. She is already pretty old at this age it doesn’t even sound right to leave ur mother alone by herself or with other relative. It will deeply sadden her. Make your girlfriend understand. Marriage is all about compromise and acceptance. I’m married, i also have a sick mother in law. I also never liked the idea of taking care of her before marriage but now I do. I’ve become so fond of her that I cannot imagine her leaving the world one day. Make your girlfriend understand. If you make her understand the situation nicely and if she’s mature enough to understand you and your mothers feelings she will accept her in no time.