How can I be a good younger brother?

You can be a good young brother by applying the following.
Be respectful: be respectful to your siblings, your parents and everyone around you. Do not talk back at them when they talk to you. Politely present your point and do the right thing.

Spend time with your siblings: spending time with them will enhance your relationship with your siblings.
Help your siblings and look out for them: help them with their work, house chores and always look out for them when they are in trouble.

Be responsible, be caring, avoid violence, be obedient and do what’s right at all times. All these tips would definitely help you to become a good younger brother.

I have never had thi question before! The very fact that you bother to ask it suggests that you are more than worthy of being a good younger brother than most. Well done for even bothering! Your sibling should be proud that this is important to you. My answer would be that you stay loyal, be ready to listen and to prepare to support your sibling.

This does not include supporting something wrong, of course. Sadly, there ar occasions when a family member will want a younger one to join in with something wrong and when the younger one hesitates, the older puts pressure saying something like 'If you were a god younger brother you would do as I ask.' That is unethical and you should not give way.