This is a matter of different narrations in Sunni and Shia books. The Prophet Muhammad (SAWA) had many fostered or adopted daughters who was looked at as his daughters according to Arab society before Islam. Many persons mixed between the real daughter (Fatimah) and the fostered and adopted daughters.

Authetuc narrations confirm that the real daughter of the Prophet and Khadija is Fatimah, while other daughters were fostered or adopted daughters. Lady Khadija (AS) had a sister Haala Bint Khowailid, who passed away leaving her daughter who were been looked after by Khadija and the Prophet (SAWA).

There is a nice book with evidence from Sunni books, written by Allama Sayyed Jafar Murtadha in this subject. بنات النبي أم ربائبه 

You can read it on this website www.mezan.net

Wassalam.