A lot of people will say "how is this a big deal"? but until you're in that situation, you may not realize that IT IS A BIG DEAL!
Especially here in the African Continent where Marriage is the ultimate achievement! My friend sent me a message asking how I coped when my younger sister got married before me, because her younger sister is getting married and she wants to handle this situation properly.
For some people there might be a bit of jealousy, or envy or that "one kind feeling" and it's okay. You want good things too. I definitely did not feel envy or jealousy but I felt like as a "Senior sister" and "first born" I did not follow schedule, so that was where my own pressure came from. My mates are married. And so? My younger sister is getting married. And So? Life na turn by turn. You have to remember that.
For Big Sis, make sure you are happy for her. Of course you are happy, but you know how emotions can sometimes get in the way of the happiness. If you cause one bit of drama, your motives can easily be misinterpreted. You don't want that. Be happy for and with your sister, enjoy helping her prepare for the big day as much as you can! In fact go out of your way to make everything stress free for her. It's also a way to "Sow" into your future.
As for lil sis getting married, be sensitive to everyone's feelings. There's no need to start unnecessary drama or reading meaning into things that are not there. Avoid snide comments and back talk. Make sure you get married with all your relationships intact! Be respectable in everything you do. If you feel your sister is acting weird, you can ask her out to lunch and talk about it.