What is the ruling on a brother assisting his unmarried brother with marriage expenses, and the resulting impact on his household expenditures, and what is the ruling on concealing this from his wife, and what is the required intention in doing so?

It is not permissible for a brother to give money to his brother if that prevents him from fulfilling his obligatory financial support (nafaqa) for his wife and children, as the financial support for the wife takes precedence. However, if he is providing for his family in an appropriate manner, then he may give his brother whatever he wishes, whether the brother is poor or rich. Some scholars have gone so far as to state that it is obligatory for a well-off brother to provide for his poor brother if the latter's financial support falls upon him. The husband is not obliged to inform his wife about what he gives to his brother, and there is no betrayal in that. Assisting one's brother financially is a righteous deed if one has fulfilled their obligatory financial support.