I believe tht we can have some of our most profound spiritual moments in our sleep, and that sometimes we can't find a rational explanation for them.
I am of the opinion that the love our spiritual family has for us does not dwindle when they cross into the light, and that once they have acknowledged they are in their new 'heavenly home' they will do what they can if they feel we need to know they are OK in their new home.
For me I would say that your sisters inner being came to you in your sleep, others would say perhaps you travelled astrally to where she was spiritually - but I think it all boils down to the same thing. The love you have for your sister, and the love she has for you, coupled with your belief that there is another existence after the one that we live day to day means that your sister was able to come to you, or you go to her, when you really needed too - and that I think is a beautiful thing.
Thank you for posting, it takes courage to jump on in with such a personal post Shell, I'm glad that you cna sense the Light and the Love in this community to share your question with us.
Love and Light
Erika xxx