Also, to what extent does this extend?
Does the OP not have to buy any Christmas presents for her SC because 'they'll get them from their mum and dad, why should they get one from me too when my DD won't get one from their mum?"
If the OP buys her DD an ice-cream when the family are in the park, does she turn round to the dad and say "well these are your kids, you can buy them an ice-cream out of YOUR money?"
It's different if we're talking grown kids who never lived with you; but if you are in a partnership with your partner, you co-parent the children in your house together, surely?
No-one would say it was OK to have one rule for the DD and another for the SC (bedtimes, behaviour, rules etc) JUST because they were 'not my children'. No-one would say it was sensible for OP to drive her DD to school but not drive the SC to the same school because they're 'not her children'. Or at least I bloody hope not. But suddenly when it's about money she can treat them like they're nothing to her, or jump through these legal hoops to ensure her own DD (their sister) is advantaged over them.
Far from uncommon; it seems many stepparents want the man/woman, but see the kids like an unwelcome side-effect to be minimised as much as possible. Especially once 'our own kids' arrive. I think this is particularly common in stepmums because it's common for kids to live for more time with their mother, so it's easy for a step-mum to see them as occasional 'visitors' rather than their husband's real children.
Some people think things should be kinder for kids who have already suffered the break-up of their family, and have to move between two different households, only having each parent part of the time whilst subsequent half-siblings have them all the time.
Always makes me laugh when such children are described as advantaged by having 'two homes' and 'two bedrooms' and 'extra parents'; believe me, what most children want is one home with one mum and one dad who love each other.
The fact so many have to do without that is sad but a necessity; the fact they are then expected to accept additional adults in their lives who have the power to take over their space and tell them what to do, but no obligation to love and support them and treat them equally in the household because they're 'not my child', is sadder still.
And the fact so many adults have absolutely zero sympathy for such children and seem to actively dislike them... well I think that's sad too. Not saying that's the case with the OP, but so many of the spiteful comments on here ('they are not your family, they are nothing to do with you, you owe them nothing, what they inherit isn't your problem') seem actively antagonistic to me.