Komitat Hont
He was born into the "gens" Csák as the youngest son of Matthew I, founder and first member of the Trencsén branch, who served as Master of the treasury (1242–1245), and Margaret from an unidentified noble family. Peter's elder brothers were Mark I, ispán ("comes") of Hont County in 1247, but there is no further information about him; Stephen I, Master of the stewards from 1275 to 1276 and from 1276 to 1279; and Matthew II, a notable general and Palatine of Hungary (1278–1280; 1282–1283). He had also a younger sister, who married to the Moravian noble Zdislav Sternberg, a loyal bannerman of the Csák clan. Their son, Stephen Sternberg (or "the Bohemian") later inherited the Csák dominion because of the absence of a direct adult male descendant after the death of Matthew III in 1321.