What is the ruling on money a young man takes from his brother who trades using the "joint sale" system? And is this money permissible for him if his brother pays on his behalf, and is it permissible to take from it when necessary, knowing that not all of his earnings are in this form?

You must investigate the source of the money your brother gives you if you suspect it is unlawful and your suspicion is justified. However, the scenario described does not give rise to suspicion; it is merely a partnership between your brother and his associate, sharing its profits. This is permissible as long as the company's activity is lawful.