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I know a lot of people are saying that Kai and Nya had a babysitter and whatever, but guess what.. who cares? Allow me to explain:
A Babysitter is someone who comes for a short amount of time to take care of kids. A babysitter needs money. This ‘babysitter’ definitely had her own family and I don’t think a random stranger or even if the babysitter was a good neighbor would raise someone else’s kids and provide for them and support them and love them for free. She most likely came in only to feed them, and left. Or left the minute Kai turned ten. Maybe she gave up when no more money was coming her way, and she only came to feed them for a year and that’s it. We don’t even have enough information to conclude any of this, especially not if she was a good, loving babysitter, which she was likely not. Because if she was, Kai and Nya would be mentally healthier.
When kids are growing, they look up to their parents, because they are emotionally connected with them. Kids mimic emotions from their parents, and look up at them for validation of their own. Nya was really young when Maya and Ray were captured. Kai was very young too, but old enough to know what love is because he most likely remembered his parents loving him. Now on the other hand Nya probably had little to no memory recall of her parents at all, much less the idea of what ‘love’ is.
When Maya and Ray were captured Kai was the only person left who Nya was emotionally connected with. Kai was the only person who could fill the role of being a loving person in her life, someone she could relay on and trust with vulnerability. I don’t think some children would just suddenly trust a random adult in their life, especially not Kai (remember how hesitant and suspicious he was when he first saw Wu). 
If this babysitter was that awesome, dependable, or trustworthy, Kai would not have trust issues, Nya would not need to feel independent, etc etc. Again, Both siblings would have a little more healthier mindsets.
Kai, as a child, loved Nya since the beginning. When Maya and Ray disappeared, he did raise her. You could interpret this sentence in many ways- raised her by love, raised her emotionally, raised her by providing for them both, raised her by protecting her. I strongly believe in every one of these and more, but this furthers on to my point: babysitter or no babysitter, at the end of the day, Kai did raise Nya. And I think for a small child who was just as lost and scared, he did his very best.
A child forms their idea of love normally by looking at their parents relationship, and by observing how parents treat them. Kai had a slight foundation in this already, since he loved Nya since the beginning. Nya however, as I mentioned before, likely had little to no memories of her parents. So, everything Nya learned about love, how to love, etc was from Kai. And with this, Nya had understood and developed her own love languages, and proceeded to use what she learned from Kai and reciprocate it. This was undoubtedly Kai’s biggest mental support during his childhood. And with what she learned from Kai, she started to also love Jay.