What can this dream possibly mean?

From what you have described it seems that when for the first time your boyfriend told you about how he and his family members were expecting a kid sister and sadly how it did'nt happen,the whole episode itself is so touching and sensitive in itself that it has so many elements and emotions attached to it like expectations and positive planning ,it even reached at such an extent that they even had decided the name,when you find yourself being completely attached with someone like your immediate family members,your boyfriend or for instance yourself then it becomes very tough for anyone to handle things like depression associated with it which can result from various things,well in this case the thing was non fulfillment of certain crucial expectation,To see somebody be gone before coming to life is very sad and heartbreaking,so on hearing about it it touched your mind and heart so much that it occupied your conscious mind and while sleeping your subconscious mind full of mixed emotions took it out in form of a dream and you started relating your chilhood with it.This is completely normal and even I had a similar dream.BEST OF LUCK 2 u............