don't always make the older one sacrifice (讓弟妹)to make the younger one happy, cause he/she will be older than that sibling for the rest of his/her life. If is not fair to ask someone to sacrifice for another person for the rest of his/her live.
If they fight or quarrel, separate them, let them calm down. Then teach them nicely ( be calm yourself) how to treat each other politely and both have to 忍讓,商量及一人行一步佢出妥協. It is the same rule in the outside world, you cannot get everything you want, cause someone else wants the same thing or do things another way.待人接物遲早要教, so it should start at home.
don't beat them because they beat each other, cause you will be teaching violence (from you) is okay in order to solve a problem. You can punish them another way, like no TV for one day, or cannot go out the next time the family goes out.