Did you watch his interview? Paul O'Keefe made it sound like his family were all suspicious at the start due to KR's behaviour.
The most odd account of her behaviour that I heard from the witness testimonies (and again from Paul's interview) was that shortly after JOK's death was confirmed, Paul O'Keefe's wife who was reasonably close with Karen, called her to check if she was doing Okay. Karen replied that she needed to remember the bad times and that she probably wouldn't be seeing them any more.
POK's wife then relayed that to the rest of the family. I can understand why they all found that behaviour to be suspicious.
I can also perhaps understand on Karen's part that she'd always felt like an outsider and that since the only thing linking her to the rest of the family (JOK) was gone, that she didn't feel that she had a place there anymore.
Also, at that point she may have believed that she'd accidentally killed JOK - "I hit him"/"Could I have hit him".
Some people display odd and unexpected behaviour in times of shock and in grief.
While KR appears to very intelligent, it's clear that she's a complete mess in the face of a crises.