You're right about it not being comparable really as if you're the one doing the cooking and care as I was (and incidentally having my things stolen by said dsd, plus all the drama - and COST she would always bring with her) it's not really just about whether the family diary is free - it's about how knackered you are, how mentally strong you are feeling, and what the reason is from Mum as to why she needs the switch.
For example - One Friday I'd done two 60 hour work weeks in a row (having my own young DD the weekend in the middle, plus DSD). We didn't have anything in the diary at all, but my god I needed that time. DSD's mum called to say she hadn't slept very well the night before and could DH have DSD for the weekend... she was a stay at home mum with ONE 12 year old DD ffs. So no sympathy.
I actually said yes - because I knew if we said no, his ex would tell DSD she wasn't wanted here and that would do nothing for our already difficult relationship with her. But the point was that my DH asked me he said he knew what a hard time I was having and would appreciate if I wanted him to say no. It made me feel like 1, he respected me. 2, he put my needs above his ex's demands, and 3, I had some say in my own home.
These men who don't ask are not respecting their wives and in my opinion are still in part married to their ex. I think particularly where they've been married a while, they slipped in to this subordinate status where they do what ex wants. Plus the guilt.. and the fear..