The Sweetest Sound I've Ever Heard

In recent developments of the incredibly cute kind, the kids have started playing together.  Nala is 19 months old and Kaio 3.5yrs.  They share toys and food with each other, interact during pretend play, and the cutest thing: tickle fights.  This happens mostly at night when i am reading to them, one of them will start laughing about something and then they will both start wrestling, tickling, and giggling hysterically.  Kaio will mimic Nala's funny laugh that sounds like a CD skipping, "Eh,Eh,Eh,Eh,Eh,Eh,Eh."  I stop reading and bask in the most beautiful sound on Earth: the happy giggle of your children.
Kaio is also falling into the big brother role.  A few days ago when we were getting ready to take a bath.  Kaio initiated some drama over the water being too hot or too cold.  Nothing would please him and he was crying and crying while Nala was complaining that I wasn't giving her enough attention.  Mari was sitting on the sofa, watching tv and offering no assistance.  I freaked out and decided that I needed to remove myself from the situation to keep from having a beak down.  I just left and closed the door to the bathroom.  Kaio stopped crying while Nala started wailing.  I could hear him consoling her, "It's ok baby, I'll open the door.  I'll get us out."  He tries to open the knob but it was too slippery, "oh no the door's locked.  It's ok. I'll get the key."
The fact that he stepped up and took the initiative to try to help get them out of the situation and calm her down just melted me.  I walked back in to calmer kids that were ready to take a bath.
Kaio has also been wanting to sleep in her bed but I don't let them because I'm worried they'll kick or suffocate during the night.  But every morning for the past week I find Kaio in he bed in the morning, so he must be getting in there after his midnight pee.
Finally one last thing, they team up on me.  Yesterday at lunch Kaio started talking about wanting to go to "Thomas Land" - a section of Six Flags park with rides from the Thomas show.  "Mom, can we go to Thomas Land on Friday?   Mom, I want to go to Thomas Land. etc"  and right behind him Nala started making giberish, with the same pleading tone, "deh deh to to do, Choo Choo, eh do de do te do."  I was like, ok so they are both begging me to go to Thomas Land now, great.  And so it starts!