Board games, manicures, movies, cookies, popcorn, fake tattoos. Let them give you a makeover, or try some home science experiments.
Take a quick drive to the library, if you can, and check out the Artemis Fowl books. You and the 12 year old can read them out loud to the six year old. The books are hilarious, and both girls will like them. Other book series to look into would be The Frog Princess (which is not the Disney version), the Clarice Bean books, or anything by Roald Dahl. They might also like the Percy Jackson books (the first in that series is called The Lightning Thief).
Have fun!