AIn the second episode of the second season, Charlie goes to talk to his mother.
In the next episode, Alex's grandmather calls to her to tell that she's coming in the house for dinner. In the verry end of the episode, the grandmather walks in and you could see that Charlie's mom and Hristo's mom (Hristo is Alex' father) are actually the same person, therefore they are brothers and Charlie is uncle to Alex, as a brother to her father.