Is there any chance you could do a practise run in the venue, or somewhere similar so you can get an idea beforehand of the kind of exposures you're going to need, and try and do any troubleshooting you might need to do before the big day itself? Or practise doing some candid shots in a similar space so you can get at least an idea in advance of what works? The problem with wedding photography is you don't get any second chances if you miss a moment, or, god forbid, the whole shoot goes wrong. If you do a practise run and struggle, at least you've got some time to work out what's going wrong.

Would definitely also recommend looking through the wedding tags on Instagram and at some pro's websites to get some ideas for poses/composition etc. Also do some work at getting off auto! If you find yourself confronted with tricky light, you're going to need that greater control to get something usuable.

Honestly, though, it is a big ask, and I wonder if your sister even realises how much of a big ask it is. I have quite a bit more experience than you, but I would still probably say no on the grounds that I've never done a wedding before. Too many people - I'm guessing including your sister - think all you need is a decent camera and that does the job for you, but that's not the case at all, not with any kind of photography, but wedding photography is a particular skill in itself. If you're determined to go ahead, I would definitely suggest having an honest chat with your sister first to make sure she is fully aware she's not going to get pro-quality shots, and she may not get anything better than your average holiday snap. And that is no offense to you, but a pro wedding photographer will have years of experience and top-notch gear that they are thoroughly familiar with.

If you do go ahead, good luck, and let us know how it goes.