Adding New Relationship Types; Is it Possible?

I've seen a few simmers talking about wanting new relationship types. Particularly, i've seen a lot of people asking for step parents/children, half siblings, and in-laws recognized as relationship types in game. For these ones, could the family tree somehow be referenced to make any of those possible or are relationships determined more directly? Or is referencing other sims' relationships part of the reason these are missing?
Another request i've seen is to be able to assign more things in CAS like, adopted status, cousins/aunts/uncles, the above mentioned.

And to Sims 4's credit, there are a lot of romantic relationship types possible in the game, but
I've also seen a few people wanting a friends with benefits type relationship for their sims
i imagine it like

• Similar to best friends, there is 1 specific, non-autonomous interaction triggers the relationship status
• Allows romantic interactions (minus profess undying love)
  
• Extra cool if romance actions occur, but without a romance bar appearing for the duration that the two sims maintain that relationship status (would make it a viable option for sims that people want to think of as aromantic)
  
• Possibly ended through another interaction, canceled if asked to be bf/gf, or ended via low friendship?
• Jealousy off/on/random could all make sense
  

But i'm wondering, is it possible for a mod to introduce a new relationship types like this, with a custom icon and description, besides just altering romance/jealousy occurrence on preexisting relationships? Are the family ones more or less possible either?
Would anyone like to experiment with the idea?