It's back to sexism and wifework though isn't it?
My mil does this, and it is purely a sexist thing. When she minds sil's children, or does her ironing, or washing, or helps her out financially buying uniforms etc, she is helping sil.
If she did the same for DH, she wouldn't be helping dh, she'd be helping me, as those are all "wife" jobs.
So she spends way more time with sil and her kids, has them in the holidays etc, and so knows them far better, has more photos. She has been known to drop me in the shit with childcare if sil phones her up to do something because sil is her priority, not me and my childcare. Childcare nothing to do with DH, obviously.
DH hates it too, as he's an equal parent and they really don't see how involved he is with the kids and how much he does, because, you know, I do it because I'm the wife. They think it's hilarious that I "have him ironing".
It's that old saying that a son is a son until he takes a wife.
It's sexist stereotyped bullshit.