Name the director to have directed most number of sister pairs.

Maniratnam.

Ambika, Radha (both in Idhayakoil)
Radhika (Pagal Nilavu), Nirosha (Agni Natchathiram)
Banupriya (Thalapathi). Shanthipriya (Anjali)
Shalini (Alaipayuthey), Shamli (Anjali)

Trivia courtesy: Kamaraj C.