My best friend got married and asked me to be my sister. Do I need a red envelope? How much do you usually pack? Was it given the day before the wedding? Stationary junction

Of course. As long as you go to a wedding or a wedding reception, you should give a red envelope, regardless of whether you are invited to be a sister.

It doesn't matter when you give it, you can give it in advance. But in addition to red envelopes, you can actually send gifts directly, and it is not limited to red envelopes. You don't have to give gifts. If you don't have a good red envelope budget, you might as well choose a meaningful gift for her.