2 theories about mr Kaplan that came to mind:
1) her 'sister' is really her.
Red found her, and decided to put her name into the witness protection list and let her retire in peace.
just like he let that girl
retire in peace.
2) her sister is really her sister.
Red's story about protecting her identity is true.
that means the person who stole his $$$ was mr Kaplan.
she killed his money launderer and made it look like a heart attack.
the more I thought about it, the more I don't believe #1.
the govt has backup copies of the list. so adding names to the stolen list does no good.
I think #2 will come into play when Red finds out that mr Kaplan is person sabotaging him.
and as show formulas go, she'll be found at her sisters place.
aka chekhov's gun.
https://en.wikipedia.org/wiki/Chekhov's_gun
"If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there."
basically, if it's mentioned, it will be used later.