Is it true that Abu Sufyan was the paternal uncle of the prophet, sallallaahu ʻalayhi wa sallam, and was breastfed by Bibi Halima and became the foster brother of Prophet Muhammad, sallallaahu ʻalayhi wa sallam?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

If you mean Abu Sufyaan ibn Al-Haarith ibn ʻAbd Al-Muttalib, he is the paternal cousin of the Prophet, sallallaahu ʻalayhi wa sallam, and not his paternal uncle. He was the foster brother of the Prophet, sallallaahu ʻalayhi wa sallam, as they were both suckled by Haleemah As-Saʻdiyyah. The paternal uncle of the Prophet, sallallaahu ʻalayhi wa sallam, who was also his foster brother was Hamzah ibn ʻAbd Al-Muttalib .

Ibn Al-Qayyim wrote:

“Then Haleemah As-Saʻdiyyah breastfed him (the Prophet, sallallaahu ʻalayhi wa sallam) along with her own son ʻAbdullaah ... and she breastfed with him his paternal cousin Abu Sufyaan ibn Al-Haarith ibn ʻAbd Al-Muttalib. He was known for his intense hostility towards the Prophet, sallallaahu ʻalayhi wa sallam, but embraced Islam in the year when Makkah was conquered, and he became a good Muslim. His paternal uncle Hamzah was also breastfed by a wet-nurse from Banu Saʻd ibn Bakr; his mother (wet-nurse from Banu Saʻd) breastfed the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, one day while he was at the house of his foster mother Haleemah. So Hamzah was the foster brother of the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, by way of two wet nurses: Thuwaybah and the woman from Banu Sa'd.” [Zaad Al-Maʻaad]

For more benefit, please refer to fatwa ">309520.

Allaah knows best.