Girl Bonding

Is there an age when kids start realizing how much they are emotional attached to something / someone? Well, no kidding, perhaps even I have not known my attachment until I missed it.
Anyway, 2 nights back Junior slept early ... around 7:30pm. (Generally she is put to bed around 9:00pm). She had got 4 shots on monday during her 4 months visit to the Pediatrician. Her body was warm and perhaps aching from the shots, so she slept early. She had been feed around 7:00pm. So around 10:30pm motherly instincts kicked in and I remembered that it had been over 3 hours and Junior needed to be fed. So I was about to pick my baby from her bed ... when BP came up and said, he would change her diapers before her feed, so she could promptly fall asleep after the feed. He gently tried to pick her... when Junior started crying. She started crying so loud that for a sec, I got quite scared that she might choke. Cantaloupe ran down the stairs from her room, looked at Junior and started crying as well. When I asked her why she was crying, she said, I just can't see my sister crying. Why do you guys make her cry ... please don't do anything to my sister.
I was quite shocked (because she wakes the poor baby every now and then and its all fine ... but now she supports the little one) and pleased. Pleased because well, I know my daughters love each other !!