For a step parent to only really sort the care for their own DC?
So say Sarah is married to Ben. Ben has three children with Jill his ex and has one child with his now wife.
Ben is a bit useless and never does things like changes the children's sheets, has clothes for them at his house, gets them to bathe regularly when there, ad-hoc brushing of teeth etc... Ben has his children 3 nights a week.
Sarah does all of the above for her child reliably, bath time every night, teeth brushed, sheets changed, new clothes all the time, room tidied etc...
Is it reasonable for Jill to be annoyed that Sarah, whilst doing this for her child with Ben, doesn't get involved in making sure it happens for his other children?