We already have one DD.
She found out last week that it (looks like) we’re expecting a boy, and revealed to me how delighted she is that they’re now going to have a grandson (from a son...they already have 3 from DH’s sister), that she’s been secretly wishing and praying for a boy, and asking her close friends to do the same (!) And the congratulatory tone to me, as though it’s some kind of achievement on my end...and presumably a failure if it had been a girl.
When we chatted after the 20 week scan with DD, she spent a good time on the phone speaking in a consoling tone about all the virtues and redeeming qualities of girls, as though it’s something that I was feeling bad about (it wasn’t!), instead of her projecting onto me.
Sure, it’s nice that we’ll hopefully have one of each gender (will probably stop at 2), but that’s nothing to do with this fixation on having some kind of male heir 
I’m fucked off at this sexism and at the forthcoming, fawning overinvestment in her grandson.