You can refer to the following Shia site – It confirms that Hz. Fatimah (ra) had 3 other sisters (biological) – Parents being Prophet (saw) and Lady Khadija (ra).
A Glance at the Life of the Holy Prophet of Islam
Khadija, peace be upon her, was the first woman who believed in the Prophet’s divine prophecy. She put all her wealth at his disposal to propagate and promote Islam. [39] Six children were born of his marriage: two sons named Qasim and Tahir who passed away as infants in Makkah and four daughers named Ruqiyah, Zaynab, Umm Kulsum, and Fatima, who was the most prominent and honoured of them all. [40]
The blessed wedding took place in the best possible way, the Messenger moved in with Lady Khadija who felt that she was going through the happiest period of her life, because she had reached her best wishes and sweetest dreams.
Khadija gave birth to several children of whom only four daughters survived: Zainab, Umme Kulthum, Ruqiya, and Fatima-Zahra who was the youngest and most exalted of them all.