Mata Sita an incarnation of Goddess Lakshmi and appeared on
earth with the sole purpose of being the wife of Sri Ram, the incarnation of
Vishnu. There is no straight answer to as who is the father of Sita in
Ramayana. This is because King Janaka had got Mata Sita while ploughing the
field.
But in most scriptures, King Janaka is considered to be the
father of Sita as he received him from Mother Earth.
It must be noted that there are several regional folk
stories which suggest that Ravana was the father of Sita but this is not based
on any scriptural authority.