As for Caroline, the the song that the turrets sing at the very end of Portal 2 begins with the words (translated from Italian, emphasis mine):
Dear beautiful, my beautiful darling!
My child, oh Chell!
For I hold her in esteem…
For I hold her in esteem.
Farewell to my dear!
As GLaDOS was clearly affected by Caroline's personality, I think that one can make the conclusion that it is Caroline who is singing this song to her child -- not to mention saving her from the turrets that appear to be about to kill her at the beginning of the above video.
There's also this painting hidden in a side room in Test Shaft 09. I would argue that that's not really the sort of portrait that an employer and his assistant would have made. Especially an assistant that Cave Johnson describes as "married to Science". It seems, to me, to imply a personal relationship of some sort.
Both of those are pretty thin, but that would be some of the logic behind concluding Caroline is Chell's mother and Cave is her father.
(There's also a drawing of Rattman's that some believe to depict Caroline (in the center, mouth wide open). That's the only connection between the two I can find. Perhaps his protection of Chell in the Lab Rat comic implies paternal instincts to some people?)