Children do not understand these consequences at 9. It is only as reason begins to develop around 11 or 12 that one can discuss these things rationally. At his age what works is simply inculcating good habits either by observing them in parents or simply as a part of habits at home. For example, it should be the norm in the house that each person picks up their own plate, don’t litter things, keep the room tidy etc. Similarly norms for watching TV, waking up in time, etc. It works best if these things are inculcated early, at 5 or even earlier. The age from 5 to 10 is the period for forming good habits. But this cannot be done by you. It is the parents who have to bring him up responsibly. Your role as an elder sister is to gently suggest, be there as a guide, play and discuss and encourage him to share things with you, shield him if he is hurt and take care when he is unwell and needs you. Of course you can pray and teach him to pray together for everyone.
As to parents, expression of love and care, discussions about various aspects of life and the world, spending time together, engaging in common activities such as evening prayer before dinner and having a meal together, outings together from time to time are all important and very helpful. See if you can suggest it to them.
Affectionately,
Alok Da
