Brother's daughter is your niece. Her marriage is undoubtedly a great happiness for your family. Congratulations!

At present, according to the custom in China, it is customary for relatives to give red envelopes when they get married, so you are no exception. You should wrap a red envelope. You should also look at what others have given you. They give red envelopes, so do you.

As for how much this red envelope is, in order not to be embarrassed or lose face, relatives of the same generation can discuss with each other, and it is better for everyone to wrap it as usual.

The above is my opinion, I hope it will help you.