Early Childhood

An early childhood memory; how do you write about an early memory with someone who is not yet here. My baby sister is still about 3 months away from birth. So how can I write about a memory? This question puzzled me at first, I was not sure how to go about it. However now I find myself writing about camping trips we have gone on while she was in my moms stomach. I tell her about all the details, all the activities, and all the fun we had.