What should I do if my brother gets a marriage certificate and my sister sends a circle of friends?

You can write some sentences to bless the couple.

You can send: I still remember that you were a naughty boy. I didn't expect to organize my own family when I grew up. My sister came to wish you a happy wedding. Or, for example, time really flies. In a blink of an eye, your brother is getting married. I believe your married life will be happy and sweet, because the bride is a good girl who understands. I wish you a happy life.

It must be exciting for my brother to get married as a sister, and I will also send friends. Besides, I want to send my best wishes to my brother's daughter-in-law.