Mildred's family members are the characters on the TV set. Their TV covers three walls and she immerses herself in their lives every day. Montag was trying to figure out what had happened to Mildred and himself. How had they grown apart? He suddenly realizes that there were three walls that had come between them and their communication. It was called the TV and the characters had become her family. He wasn't interested in them at all.
"And the uncles, the aunts, the cousins, the nieces, the nephews, that lived in those walls, the gibbering pack of tree apes that said nothing, nothing, nothing and said it loud, loud, loud." (pg 44)
Mildred watches these programs hypnotically, and the characters have become real people to her. They have developed the technology to the point that the watcher can communicate with the announcers and interact with the characters. They even send the watcher a script to participate in the story. She watched them and interacted with them all day, so they became her family.