Twiglett. I was in a similar situation when ds was born. My mother wanted to come to 'help'. I know realise that she was in the early stages of senile dementia, but didn't at the time (not infering this is true of your mother btw)
Regardless of her medical condition my mother was always hard work, and next to no help. In fact she would generate more work than she ever saved.
In the end she came when I was in hospital recovering from the section. My brother aranged it so that she was with us for a full week! He could only cope with her for two days, you understand, but I could cope, post section with a new baby and a toddler!
It was awful. Mother was unhappy and confused. I was stressed beyond belief. She was even cross with me because I wouldn't travel back with her and dh to mt brothers house (3 hours round trip in the car, a week post section!).
My advice to you is to do what you want. Be strong and be 'selfish' if you think it is right. You and your dh and ds will want time together, before you have to start taking other peoples feelings into consideration.