This is her grandson, not other people's son
?-( ?-( I am so suprise your mother say this sentence.
May be your mother cannot accept this change when you talk to her at that time. However, if I were you, I will not change my mind. You are her mother and you had to take care and educate him yourself. How about day time your mother go to your home in the future?
It is not a big problem, if your mother loves him, she will know how to handle it. Don't worry.