The word “stepmother” in place of “mother-in-law” is an anomalous English usage, not a mistranslation.
[ PROLOGUE.]
This play is call’d the Step-Mother. When first
It was presented, such a hurricane,
A tumult so uncommon interven’d,
It neither could be seen nor understood:
So taken were the people, so engag’d
By a rope-dancer!—It is now brought on
As a new piece: and he who wrote the play