The other part to that quote was "I'm sure it's not- we are going to come find you and take you away from there, so just be smart, little sister."
http://www.shreveporttimes.com/arti...0341/Mickey-Shunick-search-You-just-got-hope-
I think that they really are clinging to hope. I think that's what motivates them to get out of bed day after day without a sign of locating Mickey.
No one here can say for sure how they would act/react unless they have been in her shoes. Even then, we all react differently to tragedy.