I do not believe LE has shared with the SIL any information that if out in the open, would jeopardize the investigation or prosecution. I do not believe that LE expected the SIL to be so publicly vocal about the case, either, but in providing certain information to her, I do believe they expected it to be shared -or "leaked"- to a degree. I also believe that LE has much more information, some of which has been shared only with Mr. and Mrs. Stislicki, and some they will not share with anyone, until there is an arrest, or the case goes to trial (or there is a plea, etc.).
I suspect LE gave the SIL just enough bits and pieces to get her to talk to SG, hoping that perhaps she could talk him into at least establishing some sort of communication with them (i.e., "Hey, SG, LE tells me they already have this much dirt on you. Now is the time to talk. Maybe you can work out a deal. You can bring a lawyer with you, so your rights are not violated. Think about her family. If you didn't have anything to do with it, at least tell them that").
Sadly, I don't think this type of strategy is going to work with this particular individual. If it ever was, he would have talked by now. He would have helped bring Danielle home. The problem is, it is not the lawyers who are preventing him from sharing what he knows -if anything- with LE. It is up to him. I realize it isn't all that simple, that I am over-simplifying it, but if he felt compelled to do the right thing, he would. It has been four months.
Also, I'm not surprised that there has been no arrest. I think the prosecutors already realize this is not going to be an easy case to prove beyond a reasonable doubt, so they are being forced to take their time - while Danielle's loved ones continue to suffer each and every day. How anyone can go to sleep at night knowing he or she is inflicting so much pain on so many is beyond me.