How about showing a movie on the side of the house with a projector, or alternative, playing video games on the side of the house.
A candy bar set up would be nice. Get a bunch of different candies, place them in pretty bowls or vases with a scoop or spoon, and let them make their own goodie bags.
I personally would start at 30 seconds and see if it's hot or warm enough for you, if not you can always do another 10 or 15 seconds at a time.
If you genuinely do not want the older sister to come, you could say that you are only allowed to bring one guest with you. Mini golf isn't too pricey and is fun too, if you wanted another option.
If you want to keep the party at home you could watch a couple of movies (Netflix or rent from Redbox), a mini spa day at home, like do your nails and makeup, get your favorite food and candy.
Bowling or Mini Golf is always fun.