ChaCha, so sorry to hear about your neice. My prayers for you and your family.
I have more questions than anything.
First, where was she found? If in the home, was there any sign of the home being broken into? If someone came to the door, would she look before she opened it? How many doors into the residence? Was there any signs of disturbance in the home, even if nothing broken just misplaced? Another resource for asking questions is the mortuary that prepared her for burial. Usually they won't discuss the injuries unless the family directly asks questions. Ask because sometimes some bruises won't show up for 24-48 hours after death. Family can request a copy of the autopsy. It may or may not be released to the family, but it is worth a try (sometimes they can't release it on an active investigation.) The death certificate will also have some discription of the injuries.
Here is my reasoning for the questions. It sounds like she was hit with a blitz attack. If she was found at the entrance, then it most likely happened when she went to answer the door. If she was found away from the entrance, then either she with someone she knew or she could have been running from someone (but if she was running toward another entrance, then I would have expected more defensive injuries, when she got caught.) Even the smallest injury on the hands and arms is considered a defensive wound, so a small bruise or nick is defensive.) If she was in the habit of checking before she answered the door, then you can almost bet that it wasn't the ex or anyone else she was afraid of.
Check her cell phone records. Who has she been making calls to and receiving calls from recently? Talk to her girl friends. Family may know a lot, but everyone has things they don't want to mention to family. Has she been making complaints about the bf, or anyone else? When was her last contact with the ex? What was the attitude then?
Listen closely to the current BF's discription of events. Where was he when it happened?
Try to find out if both the knife and the cord or rope came from the home. Did police get any usable fingerprints? And were any from anyone outside the home/.
At this stage, don't rule anyone in or out.