I love my Kuya's kids so much that I feel my heart so wide open with concern for their safety and wellbeing. It's typical of Filipino families to be involved with everyone's lives, but is there a line of meddling I shouldn't cross?

Why, of course. At the end of the day, whatever you do, just make sure it's with the consent of your kuya. Even though you feel like you're just concerned with your kuya's kids, always remember that their decision will still be superior than yours when it comes to his children, precisely because he's the parent. :)