We will never photograph family and close friends for weddings. We base this on others that are very good at what they do, but have convinced us a big no no. With your questions, it sounds like you have not done a great deal of weddings and now right in the middle of a gear switch, even more chance for issues.
If you have to move forward with this, make sure you remind them in a nice way you are doing this for free, so the expectations are not beyond what you will give them. I know of several friends who shot weddings for family as a gift or a small charge and thought nothing could go wrong and it did. Issues with either the quality or missing the shot or even in one instant as explained to me by one of them, in their mind doing it for free, so subconsciously they did not try as hard as they would for a paying client, etc.
Just remember, long after the wedding is over, if something does happen, you will continue to have this person in your life and if you have an issue, it will continue to come up and could cause whatever. One of the pro's that had an issue with family offering his services, said to me if he could turn back the clock, he would have hired a pro to do the work and offered to carry their gear for the wedding and several others for free as a trade off. If you are renting or buying stuff, with just a civil ceremony and restaurant, what you will spend versus what you might have to pay for something this small, could balance out.
Again, not saying anything will go wrong or jinx you, not trying to panic you and 99.9 percent of the time, it will probably work out great. My opinion is based on past experience with friends and what has happened to them from it and what they now would do if they could do it over. Just a great deal to risk especially when it comes to the bride being your sister in law.
You should sit down with them and plan out every shot and let her make the list for you of what she wants to capture and then make sure it happens, to cover all bases. And click that posed shot 4 or 5 times to make sure you have a good one. Just to be safe. Make sure expectations are not high so when you give them great photos, you look like a hero.
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