Is it helpful for my sister to study in an American university and apply for my younger brother?

It's no use.

It is not helpful for my younger brother to apply for an American university. My younger brother will not apply for a better school in the United States because his younger sister is studying in the United States, because the requirements of the school are fixed.

The advantages of foreign relatives are rent saving, good transition period and faster integration into English-speaking society. Relatives can help you when you need help or make a decision.