get a few bags of really cheap lollypops, put them in a jar and give them away free or really cheap. (If they're for free, put a big sign on the jar that says "FREE". It'll attract other kids. Put signs up. Make sure people can easily see the stand, and make the cute little girl visible (that would be your little sister), in case someone goes, "awww what a cute little girl, lets buy some lemonade".
It also might help if you make all the lemonade in one hit. Take a jug of it outside, and when you're running low you can go inside and get some more. And then if you do it the night/day before, you can make sure you get it right, so it's not to sweet or sour or anything. And put a heap of ice in the jug, so it's nice and cold. Maybe decorate the stand a little, or something to attract attention. Also maybe call a few of your friends and get them to stop by during the day. Might make your sister happier if you don't get many customers.
Well done on being a nice sibling. That's so sweet.