Alexander was the name chosen by a young Beverly Crusher for one of the twins she imagined having with Stefan if she were to marry him when she grew up. He would have had two siblings, his twin brother Andrew, and a younger sister, Jennifer.
In 2367, she mentioned Alexander to Deanna Troi when trying to sort through her feelings for Odan, then temporarily living in William T. Riker's body. (TNG: "The Host")