She doesn't have to make her mother confess anything. Children also keep secrets from parents. She can contact the agency herself and track down her own sibling.

I like the way you think!

Iffen the maw din't want her girl finding those papers, she shouldna left 'em in the bedside table drawer whar somebody lookin' for the Vicks vaporub or heating pad might run acrost it. I think she subconsciously wanted somebody to find 'em. She is conflicted 'bout her past and is taking the chickenshit way of letting this come to light. If she really wanted to keep 'er secret, she shoulda put it up in a locked desk drawer or dresser, thet's whut I'da done. Or better yet, burn it. Thet would put an end to it.