There have been many moments recently when I think that Shyla is staying young due to the playful influence of her younger brother.
Shyla doesn't love warm weather, and she tends to lag a bit on outings with me if the sun is out. Yet, when Hachi goads her into playing, she seems like a puppy.
I think that the relationship between these two goes both ways. Shyla is helping Hachi learn to be happy in this world, and, at the same time, he is keeping Shyla more playful and silly than she would otherwise be.