The word G. bedhres “sister in law” appeared in the Gnomish Lexicon of the 1910s as a combination of the early root ᴱ√Beđ “wed” and G. ress “female relative” (GL/22); like its male counterpart G. bedhres, it seems to more generally mean “✱kinswoman by marriage”. It had a variant G. gadres where the initial element was G. gad “joint, link” (GL/36).
Neo-Sindarin: I would adapt this word as ᴺS. bethres for purposes of Neo-Sindarin, updating the initial element to my preferred root for marriage ᴹ√BES “wed” with sr becoming thr in Sindarin.
A neologism coined by Paul Strack in 2022 specifically for Eldamo, a combination of ᴺ√BES “wed” and ᴺQ. ressë “female relative”, inspired by G. bedhres.