I totally understand what you're saying... you've decided to do it, and you're looking for information/advice on how to do it... not asking for whether or not you should. I get it, but folks here are telling you to not do it for a reason, and I think you should take it seriously.
You said many key things...
1. You're a student. To me this says you're somewhat new to photography at a professional level.
2. You're not used to that kind of environment- more working in a studio. To me this says that you're going to be unprepared.
3. Your wife's bosses sister. This means if you mess this up, not only will you have ruined someone's memories but you will be putting your wife's employment in an awkward situation.
Look... I'm not going to tell you that you shouldn't try new things, and yes... EVERY wedding photographer has to start somewhere. Yes, some wedding photographers just grab a camera and get to it, and yes some of them become very successful or do very well with it. Are you going to be one of those? Maybe! But is it likely? No, not really.
I've been pretty much obsessing about photography for somewhere close to 10 years now. I'm very comfortable with it, have some very good gear, and have been to many weddings as a guest and even attended my own.

I went as a SECOND shooter for my first wedding a year and a half ago and BOY was I shocked to find out how woefully unprepared I was to handle it.
Now, some 15 weddings later... I can say that I only feel SORTA unprepared when shooting them.

Weddings are seriously complicated.
- The lighting tends to be pretty rough.
- The specific requirements of any given venue can be pretty complicated.
- You need to be coordinated with all the local staff and you need to know what you're doing or they'll get very irked with you.
- You need to make sure you're at the right places at the right times or you'll miss key shots.
- You have to understand all the "core shots" and be sure to get them, but also be sure to get some artistic shots that really make the day.
- You have heavy dependencies on reliable gear and need to have backup gear in case anything goes wrong.
- You have to be pretty damned fit... you'll be on your feet 10-14 hours easy, with very few breaks.
- You have to be really good at working with lots of different people.
- You have to be able to deal with bridezillas and momzillas if it comes up.
Oh and you have to slip all your standard photography skills in there, and have to be 100% on your game and able to switch things around in a SECOND because things move SUPER fast at weddings, and you have ZERO time to be going "gee, what f-stop should I be using here?"
I normally don't jump in on these threads and pig-pile onto the "no don't do it" crowd, but seriously... dude... your wife's bosses sister.
NOT A GOOD IDEA. NOT AT ALL.