Maybe as time goes on I can take over child care but equally I don't want to reduce my hours and work as a child minder so exW and DP keep their professional jobs and high wages
Do whatever you can to avoid this happening at all costs.
This happened to me when DP and I had children together. One nine-month maternity leave was all it took for all the children to suddenly become my main responsibility. I went back to work part time, took the hit in all ways while DP and his ex just carried on as normal knowing they had me to pick up the slack with the DC/DSC.
At the time I thought I was doing right by everyone and 'putting the kids first' like a good step mother who knows what she's getting herself into should do.
But actually I was being a mug and a martyr. It was very hard for me and my career. I ended up having to give it up completely and retrain in a different field.
Just a word of caution there for you.
Considering you're with a man who has two kids and a job that means he's away all week, you've really got the best outcome possible. By which I mean that he's not putting any of the responsibility of the childcare onto you, all you have to do is put up with MIL being at yours for a couple of hours two evenings a week. Compared to the potential disruption to you that this situation could cause, you've got off relatively lightly.
The fact is that when you're with a man with children, things will never be entirely how you want them to be. There will always be something like this, which makes you feel uncomfortable, like you're compromising your boundaries, your time, your privacy. This is life with step-children unfortunately. Two years is not too deep into a relationship to decide it's not the life for you.