With my dad and stepmum, the bulk of assets is willed to each other, with the exception of specific family heirlooms (paintings, medals, antiques, jewelry etc) which will be left to individuals.
Then the survivor, when they die, divide it equally between all the children. So technically I think my stepsibs are doing fairly well as I think my stepmum was less well off when she came into the marriage, but frankly, I don't care. We're family, and I think the good feeling is probably worth more than the cash. Something that might happen as well (although it's only a vague idea at the moment) is that they might disinherit all of us kids, on the grounds that we're old enough and ugly enough to look after ourselves, and have had loads of help over the years, and it'll all be divided equally between the grandkids, although that would involve regularly updating wills to make sure a younger child wasn't left out, which is I think why it's currently being left as 'divided between kids with the expectation it'll mostly be passed on to the grandkids'.
No idea what I'd do if I had stepkids. I quite like the idea that everyone is equal in a blended family, though, and I'd not like the idea of some kids getting a bigger share than others. That seems against the idea of family.