Is the right term to call the children of your sister wives in a polygynous marriage "stepchildren"?

I don't think there is a term in English that distinguishes "child of my spouse's from a previous marriage that ended" and "child of my husband from second concurrent wife". The English speaking world has been legally monogamous for a long time, so I don't think the situation comes up often enough for there to be a strong need for a specific term.

Note also that these terms do change meaning over time. For instance, in Emma, there is a scene where Mrs. Weston is described as walking with her "son-in-law". In modern English, this would mean a man married to one of her children. But that's not who he is...she is walking with her husband's son from his first marriage, what today we would call a step-son. Characters in Emma are also much more likely to use the term "brother" to mean not someone with whom they share a parent or two, but a man married to their sister, whom we would call a brother-in-law.

So it seems that in the past, there were even fewer words used to describe relationships through marriage than modern English speakers use today. And English still for instance, doesn't have a term to distinguish between one's spouse's sibling and one's sibling's spouse, and no term at all to label the spouse of one's spouse's sibling, and those situations come up much more often for modern English speakers than polygamous situations.