I have epilepsy and am currently going through a really shit time.
I went seizure free for over 13 years and then out of the blue I had one last August. I was devastated and handing back my licence was so, so hard. I knew I would have to go a year seizure free before getting it back, which was really challenging - but 5 weeks before I could get my licence back I had another seizure so went right back to square one again.
Losing my licence has made my life very, very hard but I would never consider lying and continuing to drive.
Personally, I would contact the DVLA and I would tell them. It sounds like your sister is in denial to be honest.
Your sister would eventually forgive you when she has come to terms with what has happened.
Is she being investigated for epilepsy seeing as she’s had another seizure?
At the end of the day, she could kill others and kill her self.
If it was my sister, if I knew I could possibly prevent her death (selfishly thinking more about her potentially dying in a crash as opposed to strangers being hurt), then I absolutely would tell the DVLA.