Bukhari

Hadith Commentary:
It is understood that if a covenant or agreement is based on truth, justice, and fairness, then it is not blameworthy; rather, it is necessary. However, in such a covenant or agreement, only mutual assistance and goodwill should be intended.
And inheritance has no connection with such a brotherhood, as that is the right of the heirs.
It is a separate matter that, in such circumstances, according to the established Shari'ah principle, the deceased has the right to make a bequest (wasiyyah).

Hadith Commentary:
In the era of ignorance (Jahiliyyah), the contract of alliance (ʿaqd ḥilf) was made for fighting against and attacking an enemy.
In Islam, there is no allowance for such covenants and pacts, as has been stated by Jabir bin Mut'im radi Allahu anhu in Sahih Muslim.
(Sahih Muslim, Fada'il al-Sahabah, Hadith: 6465(2530))
Since the narrator did not specify the exact context and circumstance of the prohibition of the contract of alliance, every kind of absolute agreement would be considered invalid in this way. Therefore, Anas radi Allahu anhu clarified that the Messenger of Allah sallallahu alayhi wa sallam established a pact of mutual cooperation between the Muhajirun and the Ansar in our house.
This means that a pact for righteous purposes and for the support of the truth can still be made.
However, the customs of Jahiliyyah—such as "your blood is our blood," "whoever you fight, we will fight," etc.—have no connection with Islam, because in them there was no distinction between right and wrong; rather, among them, the basis of agreements was tribal partisanship.
And Allah knows best.