I think it’s a mistranslation. Probably “what do you call the partner of a parent”
Well, they all consider themselves as the mother of Mori but they haven't quite grasped the relationships of humans. I do think that Rider considers Miria like a partner or co-parent because he gifted her a flower ring, perhaps that's why he asked that.
chapter 42 i guess rider learned that parents are usually married so he made and gave miria a ring. but they're probably not actually married or anything, i don't think they really know what marriage is
What is happening?!?! Why did Rider asked about "mother of married couple?" When did she become the wifey???