The brother-in-law is his wife's brother and has no direct legal relationship with the brother-in-law. If you get married, your brother needs to bear his own wedding expenses, and your brother-in-law can provide help, such as providing some useful suggestions or helping to prepare for the wedding, but he should not bear all the expenses, which is unreasonable.

Everyone's economic situation is different, and the economic relationship between families is also different, so the specific situation needs specific analysis. However, if my brother encounters financial difficulties, my brother-in-law can consider providing some financial help, but this is out of family and morality, not obligation or responsibility.

In a word, it is unreasonable for a big brother to ask his brother-in-law to pay for marriage, because it is not in line with the economic relationship between families and the nature of marriage relationship.