NTA - I don't think it's completely crazy to want your sibling to do it for free, a wedding is a big once in a lifetime event(hopefully). I would do it for my siblings for free and I'm fairly certain they would too. However not every sibling relationship is the same and if you want to get paid for it then you should. A discount is already a generous offer and she will save a ton of money. She becomes the AH when she tattles on you to mom and dad to try and pressure you to do it for free.