I decided to finally make my mind up and get down to it, nearly a year after my first post in this thread
so without any further ado, here are the Arêndron kinship terms.
A brief note of explanation. As I said in my earlier post, this system grew out of a culture in which brothers commonly remained living together, while women moved in with their husbands upon marriage, so certain mergers have occurred in a pattern that isn't quite like any existing kinship system (that I know of) but seems to me to be at least fairly plausible. Also, a man could have more than one wife but a woman could have only one husband (at a time); in Imperial times only around 5% of households were polygamous but the fraction was formerly higher.
There are seventeen basic terms for primary and secondary kin:
1. basat, father / father's brother
2. danda, mother
3. mail, brother / f's b's son
4. zilna, sister / f's b's daughter
5. vûtrin, son / brother's son of male Ego
6. shôlsa, daughter / brother's daughter of male Ego
7. krux, husband
8. shoida, wife
9. arbast, father's father
10. paslar, mother's father
11. ardan, grandmother
12. ngasta, aunt
13. fenkar, mother's brother
14. korlin, nephew / grandson
15. jeuda, niece / granddaughter
16. mailin, male cousin
17. xelûn, female cousin
with the obvious proviso that 14-17 specifically exclude individuals included in categories 3-6. Also, an older sibling's children might be around the same age as Ego, and when this occurred, they might be called mailin and xelûn, not korlin and jeuda.
Arêndron has a productive augmentative and diminutive, and these categories can be applied to some of the above terms to modify them for relative age. In particular, mailom, zilnom and zilnûn respectively mean "older brother", "older sister" and "younger sister". The diminutive of mail is mailin, the same as the word for "male cousin".
Then we have relatives by marriage. There are no unique terms for these (other than krux and shoida); instead, the above terms are extended. As this leads to ambiguity that is sometimes undesirable, a prefix hel- exists to mean "by marriage". (Please, let's not have stupid jokes about phonetic similarities. Every language has some coincidences of this type.) This prefix is never used when addressing a relative by marriage, only when talking about them.
Specifically, the extensions are as follows:
Ego of either sex would address their father's other wife (i.e. who isn't their mother) as ngasta, sister's husband as mailin, son's wife as jeuda or shôlsa, daughter's husband as korlin. The use of shôlsa for "son's wife" was primarily a term of endearment; it would be somewhat affected to refer to her as hel-shôlsa to a third person, but the term did exist and was a way of showing that you felt as emotionally close to them as to a blood daughter. Conversely, to use call one's daughter's husband vûtrin would seem disrespectfully familiar.
Male Ego would address his wife's father as fenkar, wife's mother as ngasta, wife's siblings as mailin and xelûn. Sister's husband was also mailin; older brother's wife was xelûn to an unmarried Ego, but zilna to a married Ego; younger brother's wife was zilna (or zilnûn) in either case.
Female Ego would address her husband's father as basat (but the familiar form baba, based on child-speak, she would reserve for her blood father), her husband's mother as danda (same), and her husband's father's other wife as ngasta. Her husband's sister was xelûn, but she could call her husband's brother mail if he could call her zilna (see above).