For example, I asked a question about what marriage meant in the time of Jesus / Moses / in the bible. Jesus was Jewish and the Old Testament is probably where the answer would be found. However are Christian's biblical interpretations of Jewish customs ontopic or should the question be asked else where (for this reason)? Obviously I'm not asking this question for its "ontopicness", but future questions as well.
If the question is about historical Jewish customs then the choices are good this is not the best site to ask even if there is overlap in accepted history.
If the question is clearly about a specific Christian perspective on a practice then it really doesn't matter if the practice is Jewish, early Christian or Hindu, this is the obvious place to get Christianity's commentary if anything official exists.
The problem (one of several actually) with your example question is that it was more of the former that the later pattern.