Kristy is the third daughter, so she has two sisters. The doctor isn't her father, so it must be her mother. She also has a younger brother, Mike. So three girls (Kristy and her sister), one boy, and a mother- altogether five people.
I'm guessing Mike is like a half brother, and that's why Joseph wasn't included in the family because he doesn't have family blood (while Mike still has half) Also, it doesn't matter how old her sisters are, as long as they're directly related to her.
That's my best guess. Hope this helps!
