A noun appearing as G. sion “grandson” in the Gnomish Lexicon of the 1910s (GL/67), probably related to ᴱQ. yó or yond- “son” in the Qenya Lexicon (QL/106), though where the initial s came from is unclear.

Neo-Sindarin: Since there are no other attested alternatives, I would use ᴺS. sion for “grandson” in Neo-Sindarin, as a now-obscure modification of S. -ion “son”.