Hello
I am an [unmarried] stepmum without my own children, with just one stepson.
While my OH is not as, um, tricky as the OP, he can be thoughtless. He and I both work full time in busy jobs away from the home. I am away usually three to four days a week and he is sometimes home and sometimes away.
We do our best to just share things out between us but I get cross if his assumption that I will be here for the boy means my plans are curtailed. The ex has never been able to plan anything in advance so she often leaves us to deal with school holidays etc, which we generally do, but now and then I have to put my foot down and say we need some time on our own. The boy is 14 now so we have the teen stuff to deal with and it's hard. He and his dad wind each other up and it's me who suffers.
When this builds up, as it does now and then, I tell OH that the reason the boy comes is to have contact time with his father, not for his mother to have free last minute childcare.
This weekend just gone I could see a perfect storm building - it was 'our' weekend for the boy but we had him the weekend before as ex didn't want him there, so I just knew the two of them had got the weekends muddled in their heads and yes, it turned out the ex knew it was our weekend but OH assumed it wasn't (they are marked on the calendar) because we had him last weekend. OH had several things planned in - a meet up with some mates Sat pm, he wanted to go to the motorbike show locally, he was working [from home] overnight Sat, and he flew to the US on Sunday. Much of this is not entirely compatible with having your son there.
And I knew the assumption was that I would just fill in the gaps. So I arranged to go to my sister's for the weekend. It's not unreasonable, I've not seen her for a while, I didn't have any plans, she is about to go off on a long trip and I wanted to wish her luck with it.
So, OH had to rearrange some things. To be fair, he did. He arranged a sleepover for the boy Sat night and for the ex to have him Sunday after the sleepover. He took the boy with him to the motorbike thing and he made the mate's meet up after dropping the boy at the sleepover, a bit later than originally planned.
Thus the boy had time with his father and I didn't get dragged into their constant lack of planning.
We've had a dreadful few weeks with the boy and, as I said, I work away in the week, so I get tired and in fact I felt I needed the headspace this weekend.
I do have the boy on my own tonight as it's his maths tutor evening and the ex won't allow the tutor to go to hers. This is fine. I'm working at home today as I have something I need to do this afternoon, but am off back to work tomorrow.
So, my advice is 1) remind him why the kid/s come over and 2) make your own plans!