Certainly after they’re married, as IMO those two families have absolutely merged/blended at the point.
Personally, our family has always done it before. Mainly because neither my sister nor myself are married, she’s been with her partner for 17 years and I’ve been with mine 4 - both men with pre-existing children.
We generally treat the children the same from when the relationship becomes evidently serious and it’s clear that we’ve accepted responsibility for those kids. Been together a year or so, living together - that sort of rough ballpark.
That may sound vague, as there’s no set thing in our family, however we don’t separate kids into shared or step once a relationship is obviously serious/committed.
We would say for example that my sister and her partner have 5 kids - because they do, between them. I wouldn’t say my sister has 2 and he has 5, they have 5. None of them are treated any differently than the others in her home or by our family.