Had to name change.
My DH's brother and partner (BIL & SIL) have a young child and we have two, they are similar in age and we would have liked them to be close.
We usually get on pretty well when we meet up in spite of being very different people so I often message SIL at the start / middle of the week to see if they are free at the weekend to meet up.
They usually reply with something along the lines of 'we'll have to see what we're doing, will let you know'.
Basically if they don't get a 'better offer' then they will meet up with us.
Aibu to feel that being a back up plan is not on and that if you make plans with family then it's just as important as any other plans, you don't leave them hanging for several days to see if better plans come up?