Naaaawwwwww re: the kmart story!

You could either tell her you have lost your receipt so you wont be able to do an exchange.

OR you could make a big deal about how she is such a special girl that mummy and daddy wanted just one. (So she will always be the Number One Special Girl).
Similarly you could say that daddy is lonely being the only boy in the house??? OR that this little baby boy was looking for a really special big sister and he chose her???

Good luck with however you end up telling her. Either way, she will come around and love the baby regardless. Until they are teens. Then they will hate each other regardless!