Praise be to Allah.
So long as your brother is the one who caused the accident and it resulted from his actions, then he must offer expiation and diyah (blood money) for accidental killing, after which he will be absolved of the sin. There remains what is due to Allah of expiation, and what is due to people (the heirs of the child) of diyah.
If the heir agrees to let him off, then he only has to offer expiation, which is by fasting for two consecutive months.
May Allah reward him for what he has lost of his child and his limb.
See also the answer to question no. 52809.
And Allah knows best.