Routine brother buys food and talks about it.

If so, you might as well be modest to your sister-in-law, and don't talk back. If you quarrel about this, eldest brother's heart is the hardest. You can say that the money will be returned to your eldest brother, or you can not say it (if you can't afford it for a while, you might as well not say it first). Sweet talk to your sister-in-law. Just be polite. In family relations, when it comes to money, it is difficult to deal with problems.