A military friend came back and invited me to attend his sister's wedding banquet. I was 20 years old, and my brother who came back as a soldier was 20 years old! Should I send a red envelope?

You'd better take some. People in China pay attention to this, so that your friends will have face.

There is another reason, because if your brother has a good relationship, then his sister is also your sister. Isn't it normal to give a little?

By the way, it's not up to us to decide how much to give. According to the situation in your area, it's just a little less than normal people bring gifts. It's just one meaning.