Should he know?

Last week I had a play date for my little one with a little girl and her mom and sister. As my little one played with the little girl, her mom was innocently asking him something about a sibling. He answered, "I love my sister Isabel." She (the mom, the little girl didn't pick it up) looked at me confused and said, "Isabel, I don't remember meeting her." I explained that she was born at 21 weeks and 6 days and lived for only 3 hours and 45 minutes. She was apologetic.

The thing is that my little one knows about his sister matter-of-factly. He doesn't even think twice (as I think it should be) before freely speaking of her. Is that normal? I always thought it was not only normal but beautiful. As if he almost has a bond with the sister that he can't share his life with. He always speaks of her in a positive way. I'm just questioning if I made the right decision telling him about her at such a young age.

To people whose lives have never been touched by such a loss, it may seem weird or even morbid. I want to be completely honest but, I just don't ever want him to feel the kind of pain I've had to. It's too late to change anyway.

What do you think?