From my experience as a VICTIM I was interviewed several times over a short amount of time and the SHORTEST interview was 14 hours of questioning, with no food or water offered, in a small room with two chairs and a desk and a camera aimed at me. I was the victim, and it was clearly the same room they used for all interrogations. If Sini was with LE for even two or three "interviews" and they all lasted 8 or so hours, I can easily understand the extreme exhaustion and the mentality that she has better things to focus on, like coming up with a plan for her living arrangements, for her daughters living arrangements, her job, how much time can she take off work, will work want her back, will she get her things back from LE or have to come up with more money for that, is WM spending a small fortune or has he drained their accounts on bond and legal fees and so on... Can you imagine having you child in foster care and your other one dead and calling friends and family up and saying "yeah they found her body, it looks like my husband did it, would you mind taking my other child to raise possibly indefinitely?" I mean yes that's what family is for, but the entire family is grieving, it's asking someone to clear out a room, create a home for this child that meets the standards of CPS and to have CPS coming in and out whenever they please. Sini has a TON on her mind right now... if she has said everything she knows and LE is still wanting to interview her more and she has nothing to change or add, why would she go in and put herself through it all AGAIN?
ETA
Plus, she still has the civil suit against her for the car crash and I believe there is a court date for that soon. I think that the insurance company is representing them based on the reason for the continuance for the trial, but this is another financial burden she doesn't need AND it even if WM wasn't at fault for it (no charges were pressed), it would be easy at this point to see her blaming him for this extra stress as well...