It is true.
I met her dad, my DH a year after her mum and he split.
She was, 3 and is now 24.
We had her full time for a while, as in she lives with me and her dad, in our house.
I knew he had her when we moved in together.
Its not SM bingo, it’s a fact.
I can o my imagine what I would be owed if I counted back stuff I’ve contributed to, directly and indirectly over the years.
Its point scoring and you are now tallying up what your total is in line with his now getting an inheritance.
Difference between me and you though is I have never done that, ever. I even made sure her mum got a month of two extra CS when it stopped, to help out. We paid for her university in part and living costs etc.
She now lives here, rent free, whilst she gets back on her feet after a shit time during Covid. Again, my house and as you say partially my Costs.
If my DH inherited money tomorrow from a distant aunt, I would not mentally stake claim to it. There is no tit for that here and never has been.
You said his kids, his responsibility. Fine. It’s not a wrong position to take, but it’s at odds with your current ‘part of what’s yours should now be mine’
And I didn’t turn it into a step mum problem, you did. You can’t say what you say about hair responsibilities to support when they’re his, and then just ignore that part of it.
And you can believe me or not, but it would be an elaborate tale to tell wouldn’t it.
It just makes you sound bad tbh, on all counts. He is tight, your behaviour is also questionable on different ways.