But it's very, very important not to overdo it.
Red envelopes are essential. Festival activities? Always be lively, but still be moderate. Look at your brother-in-law's economic base. It is not uncommon for weddings to be unhappy because they want red envelopes. I believe you don't want to embarrass your sister, do you?
The 9999 red envelope is a bit big. Of course, it doesn't matter if your brother-in-law is rich, or it doesn't matter if your local red envelope level is generally like this. But if not, be careful!
If you want to save yourself some face, you'd better tell your sister and brother-in-law in advance and tell him that I will ask for more red envelopes tomorrow, about how much. Don't worry, I'll ask my sister to bring the money back to you that day. If you don't believe me, I will give you this money first, and you will give me a red envelope tomorrow! So you have face, and I have face!
However, it should be noted that the wedding day is a mess, and you are not the only one who wants a red envelope. If you ask for more, others may open their mouths behind your back and embarrass the people who welcome you, which is not appropriate.
So, on the whole, I'd better ask people around me to see what level your hometown is generally. A moderate red envelope is almost enough.
By the way, one last reminder: it's not fun for my brother to do anything when my sister gets married. It's best to do less and not give trouble to my sister and brother-in-law, which is the best!