Here's a net geeks suggestion... STOP USING YOUR COMPUTER!!!!
It's been bugged to record every single thing you say and do and is remotely sending it all back to either your STBX BIL's or someone he knows computer for divorce evidence. IF your sister's STBX was in the house, or had someone else go into the house at a time he knew she would be gone, he did it with an intention. Now, whether the intent was to do her physical harm, or to gather evidence remains to be seen. More than likely it is to gain evidence against her for his divorce attorney and his case.
With the exception of the statement to stop using the computer would need to go to the sister, not the OP, I think that's worth thinking about. If there's no password to her computer, or if he knows the password, if it was the ex, he could have put keystroke tracking software on it. It's possible. Good thinking!
I know it's scary to think about all the dramatic stuff some have thought of, but gosh, anyone who has barely looked at the news knows how often people seem to think it's fine to take out their family. How many have I seen the news the last couple weeks?
It's been enough that I've actually had a conversation with my normal, average, kind husband, telling him that if he lost his job or we were divorcing, and he got overly upset about the money or whatever, that we could go live with my brother or my dad, no big deal, even my stepdad...we have many people who would take us in, it would be OK. If we were divorcing and he was freaking about child support so much so that he was thinking of killing us all, just tell me and I'd go live with any of the people already mentioned (with DS) and he wouldn't need to pay money. I'm sure there are just more being reported and perhaps there aren't MORE than usual, of these incidents but it's just being reported nationally more now, but it's freaking ME out, and I don't get freaked out by the news usually.
This situation is worth being scared about and worth sister doing something about. *someone* was in her house when no one should have been there. Someone left a phone whether accidentally or on purpose. I can tell you, if I had that situation, and it wasn't our landlady or her husband, and if it wasn't the condo association people, I would not be here at this condo right now. I'd have gone to CA to be with family, I would have hubby staying at a friend's if he coudln't get off of work. I'd be OUT, and I don't know how long I'd be gone. Someone was there that shouldn't have been, and that's scary, no matter who it was.