Oh, we had "Jim" for years. Jim lived under the water (we live on the Gulf of Mexico), his parents were killed in an explosion, and he lived with his Grandmother, unless he was staying at our house :) He could breathe underwater, make bombs, and jump really far and high - amongst other notable achievements.
Ugh, I really didn't like Jim - He was rather obnoxious - but he was a very creative result of my then 4 year old son's imagination. I literally had to set an extra place at the table on the nights that Jim joined us for dinner - thank heaven he didn't need actual food on his plate - seems our real world food made him sick to the stomach. LOL
Jim stayed with us into age 7. Though by 1st grade my son didn't talk about him to his friends.
My long way around the bushes to say that your daughter will out grow her imaginary family. She will also, eventually, learn not to "brag" about the "imaginaries" (or should I say menagerie?) to her school mates once she gets a little older.
Imaginary friends do make for fun stories to tell back to our children when they are teenagers. My son still remembers Jim - and he is 15 now...gosh, knows how old Jim is.