SM's are expected to not get overly involved in parenting their skids, lest they be overstepping their boundaries. To not get too involved with their skids emotionally because a SP is temporary, highlighted by the fact that the divorce rate of second marriages is so high. To not consider themselves as a form of parent in the skids life, as they aren't biologically connected (and BM is still alive - see various posts regarding the root definition of SP).
Yet SM's also seem to be expected to enforce BM's rules in their home, to provide the same level of care they would their own children, provide financially (or atleast not have any input in the ways shared finances with DH are spent with regard to skids), not treat their skids differently than their own children, etc, etc.
I'm not sure that I understand why it would be acceptable to diminish the role a SM plays in the lives of her skids but at the same time remind her of her great power to do damage. Why it is that SM is reminded that she is not a parent, yet is vilified for prioritizing her bonding time with her own baby over her skids. Why SM's are expected to respect and encourage BM's role in the skids life when BM doesn't seem to be expected to respect or encourage SM's role.
I think BM's, and their respective relationships with their children, are often times treated with kid gloves. And rightfully so, as they are such an important part of a child's life (understatement). But I think respect needs to be paid to SM's in return, as they can have a profound impact on a child's life as well.
You (and by extension, your kids) get what you give.