Tolkien used √SI as the basis for “near demonstratives” like “here” and “now” from very early in his writings on Elvish. The Qenya Lexicon of the 1910s had two competing roots ᴱ√HYA “this by us” with derivatives like ᴱQ. hyá “here by us” (QL/41) and ᴱ√KI “this by me” with derivative ᴱQ. tyá (< ᴱ✶kı̯-ā) “now” (QL/41, 49). Indications of the latter can be seen words in the contemporaneous Gnomish Lexicon such as G. cî {“now” >>} “here” and G. cîrin “present (place or time), modern” [gloss deleted] (GL/26). However, Tolkien also introduced a new root ᴱ√si(n) “this here by me” with derivatives like G. sî “here” and G. sith “hither” (GL/68). Revisions of Gnomish ci- word glosses indicate Tolkien was vacillating on which forms were temporal and which were spatial.
In The Etymologies Tolkien gave the root ᴹ√SI “this, here, now” with derivatives like ᴹQ. sí or sin “now” and ᴹQ. sinya/N. sein “new” (Ety/SI). The root √SI was mentioned a couple times in Tolkien’s later writings, usually glossed “this” (PE17/67; VT48/25; VT49/18) and in one place with the variant √SIN (PE17/67). This root was not entirely without competition in Tolkien’s later notes, however: in one place he gave primitive ✶khĭn- as the possible basis for Q. hí “here” and S. hí “now” in 1968 notes on demonstratives, though it appeared beside primitive ✶si- forms (VT49/34 note #21).
This root first appeared in The Etymologies of the 1930s as ᴹ√NIS “woman”, an extension of ᴹ√NĪ “female” (Ety/NIS). It also had a strengthened form ᴹ√NDIS, unglossed but apparently meaning “bride” based on its derivatives ᴹQ. indis/N. dîs of that meaning (Ety/NDIS). Unstrengthened ᴹ√NIS seems to have survived only in Quenya as the basis for ᴹQ. nis (niss-) “woman”, but this word was also blended with ✱ndis-sē to produce a longer form nisse of the same meaning.
In Tolkien’s later writings, both short Q. nís and longer nissë appeared as words for “woman” (MR/213; VT47/33) and Q. indis reappeared as well, though glossed “wife” (UT/8). As primitive forms, both unstrengthened √nis (VT47/33) and strengthened ✶ndī̆s “woman” also appeared in later writings, the latter given as the feminine equivalent of ✶[[p|n[d]ēr]] “man” (PE19/102).