Wellie
Sorry to hear about your sister. Can you ask for a post mortem or are you not her next of kin?
Death certificates are not always accurate. My dad (who was 91) died in a nursing home. He had been there about 4 weeks and the local GP had not been to see him, although the surgery was only across the road. When he died, he put the wrong cause of death on the death certificate. I queried it but was told it had to stand. The coroner decided that although the GP hadn't visited and in theory it should have been classed as a sudden death, he would just read the hospital notes and speak to his previous GP, and dispense with a postmortem, but we could have asked for one if we'd wanted to. Obviously, due to his age and ill health anyway, we didn't bother.
I hope you manage to get things sorted to your satisfaction.
All the best
Liz