Historically, boys were valued above girls because property was inherited through the male line, girls had to be married off with a suitable dowry, so they were seen as an expense, an outlay that brought no returns.
It is shockingly sad that some parents, not only mothers, fathers too, still have these antiquated notions.
It is also sad when modern women don't want boys, but are delighted to be the mother of daughters.
Give your DIL time to get over the shock of a premature birth and a baby fighting to live. Most mothers do manage to love the children they have, rather than those they thought they wanted.
Right now, you love this wee lassie and it sounds as if her Daddy is happy with his little daughter, as well. Plenty of Daddy's girls never let it both them that mummy preferred their brothers.
All best wishes for your little granddaughter, she isn't all that premature - I was a seven-months baby weighing 3 lbs at birth and the midwife, according to my mother, said I was still-born, until I started yelling. Best day's work I ever did. Hope your granddaughter is just as stubborn.