He was Tutankhamen father or uncle
This is another mystery historians have not particularly overcome. However, it is common to read narratives that assume that Tutankhamen was the son of Akhenaten by a mistress or a younger wife.
Akhenaten’s main queen was the mother to most of his children. But Akhenaten’s supposed parentage of Tutankhamen is still strong.
Indeed, it is recorded that Tutankhamen used to be called Tutankhaten.