You have a valid concern. If your husband is always helping his brother monetarily, he is enabling him to rely on him and not to come up with the money on his own.
If your husband is not listening to you, there is really not much you can do as he will believe continue to believe that he can do whatever he wants with his own money.
In the event that the two of you have a joint account or you believe that the money spent by his brother would have benefited your own family, you would need to let him know.
If this is possible, you could inquire from your brother in law about his financial situation and let him know that it is affecting your marriage?
However, your husband may misinterpret this and once again say that you are being controlling.
It may be helpful to have some family counseling in order to work on communication w/ him. In relationships, whatever one of the partners does will aways affect the other person.