I don't recall ever seeing a lengthy write-up of either Ranma/Natsumi or Ranma/Kurumi. I really don't read anything new in the fandom anymore but when I still did, it wasn't hard to notice that those two girls appeared in very few stories overall. Perhaps writers just didn't like them, or didn't have use for them if they did like them; or perhaps the writers weren't as familiar with the OVA's as they were with the general TV series and/or the movies.
It could be any of the above. It could be all of the above.
Incidentally, I seem to recall reading that the page in the scanned manga where Soun says he will take Ranma as his wife if Ranma never gets unlocked, is a total mistranslation. As I can't read Japanese I don't know whether or not this is true, but what I recall hearing is that the real text says something like "Clan Tendou will arrange a marriage for you as a bride".
Back on topic, if someone were going to write this story seriously, one of the things that needs to be first decided is how the girls are Soun's daughters, because from what I recall when I saw that OVA a long long time ago, they're not. Since we know Soun was married and the Tendou household is extremely large, it seems possible the family is wealthy (it not rich in cash). Maybe the family owns a lot of property; I've posited that in the past. Perhaps Natsumi and Kurumi's mother was Soun's mistress. It doesn't seem terribly likely but a good writer could probably manage it.
