@Useronemillionandthree I've name changed to say you should absolutely not fall into the role of de facto childminder, unless YOU want to. Also, don't be guilted into anything.
I'm in a similar position in that I have been largely ignored, with very superficial interest in me in the last 25 years of being a step mum. No specific issues - just pretty much a superficial relationship - it was made clear that I was irrelevant; so I just got on with things and was always polite etc.
Now that two grandchildren have come along in quick succession there is a sudden flurry of contact and an expressed desire to see more of us - followed of course by requests to have the toddlers for the weekend. Our step daughter asked my DH, not me - he has said no since he works such long hours.
We now find ourselves with a huge elephant in the room: they hint that I am now "granny" and I can see with every visit they are wondering why I am kind and nice, but in no way fawning or asking to look after toddlers, especially as I work full time.
My step daughter has not asked me directly - I think she must realise that it's quite obvious she is only being nice now because she has had children and wants babysitters - and before couldn't care less whether I was there or not.
No judgement from me - you have a mother, you don't need a second mother when you get a step mother BUT, having put no effort in for more than 25 years, neither should you expect that I should want to make an effort now.
I save my energy for people who care about me, not people who are only interested when I can do something for them - family or not!