Sorry for the delay in responding!
Well, we can't really know for sure whether your sister is in Heaven or Purgatory. While sometimes revelations come from dreams, that is exceedingly rare.
There are perfectly natural explanations for such dreams. Even if the dream was an authentic revelation, there is no certainty that it is right to interpret it to mean that she is in Heaven.
It doesn't hurt to continue praying for her. Such prayer is an act of love and I'm sure your sister would appreciate it regardless.