Have any of you done work for family?
Did you give them any kind of discount?
How did you handle support after launch?
Never mind they know it would have taken hours for them to do so without your help. This happens less and less when you can educate people about how much work it is to do a thing, exactly what goes into it, and what small things can be done to save you (them) from wasting lots of time on unnecessary work that may not be obviously unnecessary to them.
How much do you like your sister? Do you think she can afford to pay for ongoing support? Is it a successful business, or more fledgling? Is what she needs now so big, in comparison to what you've already done for free, that you feel you have to charge for it where you didn't before?
If you built it sanely, hopefully there is a way you can show how to do what needs to be done now. Normal people are usually afraid of making websites, and it's not that hard. You can do your part by making good backups of what you've already done and being available in case something goes wrong and it gets too hairy for a novice to fix. It should not be necessary for you to be responsible for editing copy on an ongoing basis.
Then again, depending on how hard-up you are for work...
And the issues: once I had a family member asking for way too much and taking way too long to respond. I had to stop doing work with that person.
tl;dr:
1.- Avoid it as much as possible.
2.- Ask for more.
3.- Read point one.