Imho, it may be as simple as who requested first.
im looking after my great niece as emergency cover from time to time. It means I have to drop my other activities those days/weeks, and frankly it’s harder work for me now aged 60 plus than when I was mum to 2ds
so if one of my own sons has a child, (they’re a while off to pint great niece will be in pre school by then) and I agreed to child mind regularly, it’d be first come first served . If then other gc came along, I’d have to really think if I’d take an extra child on . Siblings ? Probably as they’re used to each other …and parents want younger one looked after same as older one and I’d be used to parenteral preferences and rules. But adding a new child, of my second son, jeez I’d be bloody exhausted in trying to remember which child needed what, should have this, as well as trying to handle 2 plus kids that weren’t siblings.
im not a trained childminder. Plus I’m not exactly overwhelmed by babies….give me a 2.5-9 year ld and I’m far happier and a much better “parent”. I find babies really hard and frankly a lot of drudge work . Love them as I do when they’re my great niece, or my own DS.
and I certainly not be offering up more different days to the second family..I’d be exhausted, and not have time to do things I wanted. When childminding for the gc or that next generation becomes something exhausting and not enjoyable, I’d offer to pay towards childcare tbh. I wouldn’t be stepping up to do more hours to be “fair”.
The only other way would to be negotiate with both families to agree to split the same amount of time between them, or first family send their older kids to nursery. But heck, that’s very disruptive and needs a lot of advanced warning for them to get child care sorted almost as soon as you became pregnant etc.
so, please don’t read this as being unfair, favoured daughter vs unflavoured DIL. That may play a small part, as mums and daughters generally are closer, but it may just be they cannot take on any more and it was first come first served.
Has your husband actually asked his parents as to why ?