The pronoun probably goes back to PWS *himā with the extension through the deictic elements -n-t-u, present in other types of Geez pronouns. The shift *h- > ˀ is irregular (Kogan 2009:68).
PS
*šina - they (fem.)
(HALOT 252, Kogan 2009:68, Huehnergard 2019:51–52)
For the distribution of the initial š- and h- in individual languages see *šū̆/šū̆ˀ/šū̆wa (compare also the initial ˀ- in BArm). See Kogan 2009:66, 68.