As a bit of background (posted about it before) my OH and DP(arents) are NC with each other.
Im currently on mat leave and usually make time to see DPs once a week for most of the day (5-6hours) when my DM is off work.
She usually texts me the days she's off and we go from there.
It's never weekends as that's usually our "family time" with OH and the DCs and we usually do stuff together.
So my problem is on weeks like this one where she's off on 2 weekdays DC1 is at nursery and on Saturday which technically we are free on but its not a great time for us as mentioned above.
I don't want to go into the reasons we can't come this week as it will just lead to tension and conflict as DM despises OH and it will be all "you never come at weekends" "you've got the whole weekend for family time" etc or just sulks/moodiness and I'm sure she's keeping a mental note of these things to pull out at some point.
But "sorry that doesn't work for us" doesn't sit with them and there's "why not, just tell me" "why can't you do it" etc and if I just say it on repeat its seen as very rude (different language so comes across a bit different).
How do I assert myself without being rude or leading to tension?