There is already a mod for it.
As far as I could tell, I could only tag NPC's as relatives to other NPC's ("Dude A = Girl 9F's Brother") but not their relationship values to each other in terms of attractiveness/like.
This is already the case for all of my mods, I included sub/dom stats nearly everywhere and also used included NTR situtations when vin_NTR is enabled. Also, Better Pregnancy makes NPC/NPC pregnancies without Player interaction possible.
The stats are there, yes. But the scenes still play out just like any other scene, no? once the "sex scene" triggers, you, the player, still is in full control of what happens. You could end up finishing just cuddling or something like that even if both parties are super lustful etc. That's what I meant with my example; the player shouldn't have such control.
In your mods (Better Pregnancy specifically) an NPC could indeed become pregnant from another NPC. However, it is a very buggy mod. For instance, do the whole "good girl gone bad" event when you're a female player. After a few days one of the female contestants (iris for example) could become pregnant and then confronts you with it. You can't even say "Clearly not mine because I'm also a girl, and we didn't even have sex in the first place, we just cuddled". You're forced to admit that it's your kid or "it doesn't matter I'll take care of it anyway". I'm just friends - nay, acquaintance - with Iris.
It's missing a lot of variables that would make it a bit more realistic.