As far as we know, gendered terms are lexicalised in Klingon, similar to how they are in English. In English, you can say "waiter" and "waitress", "actor" and "actress", but not "writeress" or "speakeress". In normal conversation you'd say "peacock" and "peahen", but not "peafowl".
jabwI' is gender-neutral in Klingon. You could say jabwI' be', but that'd sound a bit like saying "writeress" or "writerwoman" in English. It's not impossible and it'd be understood, but it's not the expected way to say it.
Also, there is no known word nI' meaning "sibling", only loDnI' and be'nI' for "brother" and "sister", respectively. (We can speculate that perhaps there was such a word in the past, but it isn't found in the dictionary now.)