Does blood money become obligatory upon my brother who caused a car accident that led to the death of a person, knowing that the deceased acquitted my brother before his death?

If your brother caused the accident that led to the man's death, then he is liable for the expiation of accidental killing, which is fasting for two consecutive months. He is also liable for the blood money (diyya) of the deceased, which is borne by the 'Aqila (male relatives on the father's side), and he must pay his share of it as an individual among them.

The fact that the injuries resulted from another car does not absolve him of responsibility, as long as he was the primary cause.

If the victim was suffering from a life-threatening illness before the accident and forgave your brother, then the matter rests with the guardians of the blood (Awliya' ad-Dam): either they uphold his forgiveness, in which case there is nothing upon your brother except the expiation, or they demand the blood money.