Congrats! Next best thing to having your own kids. You get to be "Auntie" who spoils them rotten and sends them back to Mom and Dad. "But Mommy, Auntie lets me stay up late/eat candy/buys me anything I want at Toys R US".
I became a ready-made aunt when I married DH; his brother had another daughter in May. It's nice being an aunt. And I get to tell my nieces what to do when their parents aren't around.