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.