How to add new Family relationship Son/Daughter-in-Law?

[travelg]

I tried to find in documentation but could not find it? Can someone please tell me how to add new Family relationship which is not exist by default for example Son-in-Law or Daughter-in-Law? Thanks.

[MHohenberg]

probable duplicate to #4689

[beemej]

I'm very interested in learning how to do this as well. son/daughter in law and siblings in law and parents in law are as frequent relationships as the rest of the possible choices (more than many)