Please accept my sincere condolences concerning your loss. Things may have changes since my day but I don't think they will ahve changed that much.
I will assume that this is in England as there are different arrangements in other parts of the UK.
Where a death happens unexpectedly in England particularly out of a hospital environment or wher no GP has had recent involvement there will always be an inquest held to try and establish cause. The local Coroner's Office is the place to start making enquiries and you will find that easily on line or in the local Phone directory.
Your sister will be able to attend and if they were close friend she might even be asked offer testimony.
Post Mortems cannot always identify actual cause of death but can often point to likely cause.
I hope this helps you.