He's not trying to do anything. He's just curious. :) He loves her and wants to interact with her. I'd frame it in a way so that it isn't personal. Something like, "Now isn't the best time to play with DD." And teach him about how babies learn and develop. "I know you want to help DD, but sometimes we need to let her figure things out for herself. This is how she learns." and turn that into a conversation about how you interacted with him when he was that age.Â
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I know it's not the same, but I have a 13 month old and a home-based preschool of 4 year olds and sometimes they need reminders about what is safe for babies, and how to respect every person's space, no matter their age.Â
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Another thing you can do with DS is sit with him while DD is occupied and tell him, "Let's watch your sister for a minute." and talk to him about what she is doing and what you interpret that to mean. If she makes a sound you can ask him, "What do you think she is feeling right now?" and make that a regular thing. Maybe once a week, or more if you can. This practice not only will help your DS develop a sense of his sister's need for space, but will also help build his emotional vocabulary and empathetic sense in general.
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I think you may be interested in the book Roots of Empathy by Mary Gordon.Â
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Good luck.