@pudnanewell1986 I’m so sorry for your loss.
I think perhaps it would be beneficial to seek some professional counseling for you and your wife?
Also, I don’t know if this can be of any reconciliation... My son sometimes also play by the mirror, now (he’s three) and when he was younger. We’re pretty sure he’s been a single child, so no twin there.
He’s also really good at playing by himself and that’s usually something other parents envy - and the kindergarten claims it’s a healthy thing although his extrovert mother sometimes wonder
It’s quite possible your son has some sort of soul mate / invisible friend / guardian angel - but it may not be a negative thing, like something he is missing. It could be a thing that gives him joy? He’ll never be alone.