Muslim

Hadith Commentary: Benefits and Issues: Killed with me:
That is, he was martyred while supporting and aiding me,
because his brother Haram bin Milhan radi Allahu anhu was martyred in the incident of Bi’r Ma‘unah,
in which the Messenger of Allah sallallahu alayhi wa sallam was not present.
He (sallallahu alayhi wa sallam) would also take a midday nap (qaylulah) at the house of his sister, Umm Haram radi Allahu anha,
and this was also for the same reason.

Hadith Commentary:
Those seventy reciters and preachers among the Companions (radi Allahu anhum), whom the tribes of Ri'l, Dhakwan, and others treacherously martyred, among them the very first martyr was this Hazrat Haram bin Milhan (radi Allahu anhu).
The scholars have also mentioned that Umm Sulaym was his foster maternal aunt.
Imam Nawawi (rahimahullah) states: "It is established that she was a mahram (unmarriageable kin) for him (sallallahu alayhi wa sallam), but there is a difference of opinion regarding the manner of this relationship. Ibn Abd al-Barr and others have said that one of his (sallallahu alayhi wa sallam) aunts through suckling (fosterage) was Umm Sulaym. Others have said rather, she was the aunt of his father or his grandfather, because the mother of Abdul Muttalib was from Banu Najjar." (Nawawi)
That is, Umm Sulaym was a mahram for him (sallallahu alayhi wa sallam); some people have described her as his aunt, and also as a foster (milk) aunt.
Some say she was the aunt of his noble father or his grandfather, because the mother of Abdul Muttalib was from Banu Najjar.

Hadith Commentary:
➊
The real brother of Umm Sulaym (radi Allahu anha), Haram ibn Milhan (radi Allahu anhu), was martyred at Bi’r Ma‘unah along with seventy Qurra’ from the Ansar, which had a lasting effect on Umm Sulaym (radi Allahu anha). Out of sympathy for her, the Prophet (sallallahu alayhi wa sallam) would often visit her home. Umm Sulaym (radi Allahu anha) was also the foster maternal aunt of the Messenger of Allah (sallallahu alayhi wa sallam).

➋
If it is permissible to express sympathy with the family of a deceased warrior (ghazi), then it is even more permissible to show sympathy to the family of a living warrior. This is the very purpose of the chapter heading established by Imam Bukhari (rahimahullah). (Umdat al-Qari: 160/10) Since the Messenger of Allah (sallallahu alayhi wa sallam) himself had sent them for preaching at the request of the polytheists, he described their martyrdom as if he himself had been martyred along with them. And Allah knows best.