''Wow I've never looked at DH as being free child care, rather as DDs (his SDs) parent.''
But it all depends on the individual families and what they want.
My DH plays a parental role to my DS because he has been around since he was a baby and sees little of his bio-dad, he is a dad to him in all senses other than biologically. However, if he had a father who was playing an active parental role in his life I doubt that he would be playing the role that he is.
However, DS's Dad's girlfriend (who he's been with since DS was a baby) plays very little role in DS's life, she would never even look after him alone, just because there would never ever be a situation where there was a need for her to and she doesn't want that role and I wouldn't want it either.
I will sometimes look after DH's kids if, say one child wants to go with DH to the shops but the other doesn't etc, but I wouldn't be an actual source of childcare, unless it was for emergencies. It happened for a while in the past but I won't do it any more.
The difference in the situations though, is that DH is stepping in and playing a father role where one is absent.
With DH's children they have two full parents and two step parents, they don't need four full time parents.