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?

It is normal to be concerned with your niece/nephew. But please ensure that you also follow their own family rules as a sign of respect to your brother, and will also avoid confusion on the kids' end. Also, while you can correct the kids if they do something bad, I still believe that the parents should be the one to discipline them (if there are consequences like timeouts).