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 ‘alayhi wa sallam, is His slave and Messenger.
The story of Umm Haraam bint Milhaan was reported by Al-Bukhari and Muslim from Anas ibn Maalik . The latter said,
"The Prophet used to enter upon Umm Haraam bint Milhaan and she used to feed him. She was the wife of 'Ubaadah ibn As-Saamit . Once, the Prophet entered upon her and she gave him food, and then she started looking for any lice in his head. He slept and when he woke up he was laughing. She said, ‘I said, ‘Why are you laughing?’ He, sallallaahu ‘alayhi wa sallam, replied: ‘Some people from my nation where shown to me as fighting in the cause of Allaah, they sail on this sea like kings on thrones.’ She said, ‘I said, ‘O Prophet of Allaah, supplicate Allaah to make me amongst them.’ So, the Prophet supplicated for her to this effect. Then he put his head down and slept again.
“He woke up and he was laughing again. She said, ‘I asked him again, ‘Why are you laughing?’ He replied: ‘Some people from my nation where shown to me fighting in the cause of Allaah…’ (He said exactly what he had said the first time). She said, ‘I said, ‘O Prophet of Allaah, supplicate Allaah to make me amongst them.’ He replied: ‘You are amongst the first group (he saw in the dream).’
“So, she travelled by sea during the reign of Mu'aawiyah ibn Abi Sufyaan and when she was on the land she fell off her horse and died."
As to her relationship with the Prophet, sallallaahu ‘alayhi wa sallam, Imaam An-Nawawi said,
“The scholars agreed that she was a Mahram (a female non-marriageable relative) to him, but they differed about her exact relation with him. Ibn 'Abdul-Barr and others are of the view that she was his maternal aunt by suckling; however, some other scholars said, 'She was a maternal aunt of his father or grandfather, as the mother of 'Abdul Muttalib, was from the tribe of An-Najjaar (the same tribe as Umm Haraam).'
He further said about her and her sister Umm Sulaym both, “They were maternal aunts of the Prophet, sallallaahu ‘alayhi wa sallam, and his Mahaarim (non-marriageable relatives), either from suckling relationship or by family lineages so he was permitted to be in seclusion with them.”
So, this explains the Prophet, sallallaahu ‘alayhi wa sallam, entering her house and her looking for lice in his head.
Allaah Knows best.