To My knowledge Santhoshimatha is not worshipped as Pillaiyar or Murugan's siter.. Santhoshimatha is "Pillaiyar's" Daughter and in North pillaiyar is considered to have two wives(siddhi and Buddhdhi) and Murugan as a Bacheolor(particularly in Bengal)
You have a temple of Santhoshimatha in Chennai at Virugambakkam right on Arcot Road.