I also find dreams fascinating!
Nobody can tell you (not even a professional) what your sister's dream "really" means...only your sister can. The dream can depend on so many things that nobody knows but she - how she is currently feeling, how she feels about you, how she feels about cars, what she was doing the day before the dream and if it concerned cars...
But I completely agree with everyone here in that death in dreams is not what you might necessarily think it means. It usually means that something has finished and something new can now move on. Something may have "run" its course.
If this is not her first baby, she might be thinking about siblings and how the other child might react...
If this is her first baby, perhaps she's thinking of her own childhood coming to an end, and her nostalgic memories with her sister....thus the suggestion to herself to give you a call.
Anyway, you might ask your sister next time what she felt (alarmed, scared, angry, happy, confused, relieved) after seeing that dream. She might be feeling anxious about her unborn baby and just wants to talk to her sister for some reassurance.
....if she had no feeling this or that way about the dream, she may have been watching something on TV the night before and it simply got mixed into her dream. LOL.
ETA : It just occurred to me that it was July 4th when your sister had this dream. She may have heard a firework go off in the middle of the night that sounded like a "crash"? Or perhaps she has a memory of something you two did on the 4th of July?
Sorry I can't stop, I love to talk about dreams!