Hey there Raindropvampire
Just popping in to let you know I feel for you and you are not alone with this experience. Mine was a really close friend and just like you I dream that she has been hiding somewhere, it's always such a relief to find her and we always have a GREAT time together....then I wake up....every time it takes me a while to realise it was only a dream.
I don't know if this will help you at all but it has certainly helped me...hardly ever have those dreams anymore. I looked at what I loved and admired in my friend and I keep that alive with me....does that make any sense? As an example, she LOVED dancing, so anytime I have a boogie I imagine she's there grooving along to the music as well....never know she may well be!
I'm here if you ever want to PM me to talk about how you're doing and/or your sister...massive hugs 101