These are three year olds and, kindly, OP, it's just a playdate.
People aren't going to organise childcare for a 3yo's playdate unless it suits them. If it doesn't suit you to hang out with this family, best thing is to find other families to hang out with where the dynamic works better for you, maybe ones where there are no older siblings?
If it is important to you that your child plays with THIS child, then you're just going to have to tolerate the sibling because it suits their family dynamic to have the mum take both children out at once at the weekend (for whatever reason, dad working, dad lazy, parents split the weekend to give each one time "off"). You can't really expect people to change their weekend plans in a way that inconveniences them for a playdate.
I do get you - 7yos can be very annoying! If your child isn't at school yet, they're huge by comparison and yet they're often still boisterous, have few social graces and aren't very sensible.
I have a toddler and a 7yo. We don't do many playdates with the toddler's friends, partly because 7yos are a bit of a shock if your experience of children is limited to tinies. Instead, most of our playdates are with the 7yo's friend's families, where there tend to be a range of children of different ages. Either I drop the 7yo and run or, if I know the parents well enough that they're happy for us to stay, the toddler tags along and hangs out with the "big" crew.
We'd organise childcare for a child's party, if at all possible, but not for a playdate.