There could be a further back story here.
The op was only 19 when she met her husband, the child was three and in the fathers sole custody, which would indicate possibly the grandmother was heavily involved in raising rhe child as the father worked.
In addition if we assume it's a couple of years after meeting before the op and the husband decide to become official and she becomes mum, the gran could have been raising this child pretty much till the age of five.
If she's then continued to do childcare as the op and her husband both work, then the grandmother could very well be a mother figure to this boy.
If That's thr case it could explain the ops Sour feeling that she's not respected as mum, and why the father wouldn't cause a rift with his mother due to what she's done for the boy and their relationship, and certainly not over fish and chips. This woman may have basically raised him from a baby and their relationship may be much more than a normal grandparent grand child one.
However on the flip side if that is the case, then it would be very cruel for her to take him away from his sleep over with his gran.
That's not love. You put your child first as a parent. And in this instance the op has failed to do that.