My experiences with the Kindle are following. Use them as something to consider not as something you have to do
I do not have the warranty, in my experience if something does not break the first year then it will pretty likely last a long time.
As for the charger, you can use a standard USB charger, so if your sister already got one, e.g., with a smartphone, you can save money on it towards the warranty or the case. You can use an USB port of a computer to charge it, but it takes much longer, I'd recommend to get a standalone USB charger.
And the case, it's a good thing to get the original one, but it will cost you the arm and the leg for a bit of leather and a LED light. There are cheaper alternatives if you are on a tight budget. I have an original leather case for Kindle Keyboard without light that is probably not available anymore as there was a technical issue with them. Anyway, I use the case only for transportation and read without the case. Which means it's lighter and more comfortable to hold in hand. So you may consider that, too and get something cheaper that will protect the Kindle when in a bag, but won't be used otherwise. But the case you want should be OK, too. It's not such a huge difference in weight after all, in absolute figures. But it almost doubles the weight.