There is no statute of limitations for filing NEW murder charges. Murder is one of the crimes where LE can bring NEW charges on someone whenever they have enough evidence to do so.
I think the confusion is because she has already been charged with kidnapping type charges. So once someone is charged with any crime, then for that crime's charges, certain timing rules go into affect related to a speedy trial which the Defense can elect to "waive" the speedy trial limitation.
The timing rules for her current charges remain in effect but for NEW murder charges, those can come whenever LE is ready even if years from now.
Hope this helps.
ETA - Just wanted to add that it does get real confusing because I remember seeing that article that talked about having 90 days to file charges and I believe they were referring to another rule where once someone is ARRESTED for lets say Crime #2 (murder), then I think there may be another rule that LE has to then file official murder charges within that 90 day time frame or something like that.
But so long as she has not been arrested for Murder yet, then I dont think LE has any limits currently on how long they can wait to finally charge her with murder. There is no statute of limitations for murder so those can come years down the line if needed.
I hope I have this right and I probably should have let one of our lawyer posters answer as this does get real confusing doesnt it. LOL