Sweet, sweet sisters playing peek-a-boo can establish fond memories between them forever. There is a relationship bond forming that is built with trust, fun, and an overall intrinsic motivation to keep it all going.
They are naturally looking for each other, taking turns with finding each other, and waiting patiently to be found. This very first playful game that parents may initiate earlier in infancy, can grow with them into their toddler/preschool years.
This idea of understanding that when someone is not seen and then appears again and again and again is comforting and has a reassurance feeling.
When an infant or toddler doesn't notice the parent/family member right away, I encourage you to continue playing and provide a different way to enter back into your babe's view.
We can always gently encourage and gauge our babe's interest in the game, while knowing the importance of connecting.