Inheritance kids and step kids

I am a step-child. I don't expect to receive anything from my step-mum (and that is with an excellent adult relationship), but I would expect my dad's legacy to be split between all of his children.

ie if they both died at the same time my half sister would get 50% of their estate as she's my stepmums only child but then we'd each get equal shares of my dad's bit.

I think it depends how long you were all together. If you became a merged family 30 years ago when you were toddlers and all grew up together as one family, I can see why the “parents” might regard all children as equal.
If the families merged as dcs approached adulthood, then that would feel different.

But in the end, people can leave money to whomever they want, and it’s not really anyone else’s concern.

Children should get the inheritance their parent raised after the step parent has died ( if still alive).

We moved in together and married after my 2 daughters and his son had left home. If there's anything left, it will be split 3 ways.