Is it true that Muhammad, sallallaahu 'alayhi wa sallam, was Khadija's, may Allaah be pleased with her, uncle's son?

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 is His slave and Messenger. 

The Prophet was a (distant) cousin of Khadeejah as their lineage meets at their grandfather: Qusayy ibn Kilaab.

The Prophet's biography authored by Ibn Is-haq reads, “The first woman whom the Messenger of Allaah married was Khadeejah bint Khuwaylid ibn Asad ibn Abdul ‘Uzza ibn Qusayy.”

The Prophet was the son of ‘Abdullaah ibn ‘Abdul-Muttalib ibn Abd Manaaf ibn Qusayy.

Allaah knows best.