The tragic fate of the Trevor family is an article in Resident Evil Recollections. It was uploaded on 22 December 2014.
George Trevor, the architect of the Arklay laboratory (aka the mansion) in Resident Evil, suffered a fate worse than death in order to protect the secrets of his creation, not even his wife and daughter Lisa were spared.
If you want to know the details of their story remember to seek out the in-game files!
As it turns out Lisa isn't mindlessly roaming the grounds, she's searching for her mother, trapped in a body that cannot die.