We have 2 other cats that she loves to play with, one of them uses the litter box, the other one prefers to go outside. I make sure the box is scooped twice a day, so it's not a dirty box. The older 2 have never messed in the house. I'm afraid as a mere mortal I don't understand.
Cats are very particular about their territories and about their bathroom habits.
When they use the box they leave behind a lot of their smell, so that using the box serves as a way to mark territory. Just because your kitten and the older cats seem to like each other doesn't mean that the kitten feels safe enough to trespass on the litter box. I would suggest that you get the baby a box of its own and put this box in your bedroom where it has been going anyway. When the kitten grows up and becomes more secure in your household, it may begin using the box with your other cat. For now, it just needs a little help.