Talk to them about giving respect not only to each other but also to their belongings. Start from the first step and tell them how to take care of their toys and books etc and then teach them how to behave in front of their siblings.
Address them in a proper way. While talking to them, use respectful words like please, thank you, excuse me etc. Kid’s mind is just like a blank board whatever you will write on it, will remain forever.
Encourage them when they do use those good manners and they are saying please and thank you in conversations. You may reward them or at least appreciate them by referring to your spouse and other family members. Show them that these small things will earn great respect and love for them.
Make sure that you behave exactly the way you want them to act. Do not direct them harshly rather give them a demo and then ask politely. For example, you may show them how to pull a chair without making noise on the dinner table and then ask them to follow. Practice good manners everyday at home. Explain why are you doing this as it will be helpful for your child to understand and he will follow you.
Discuss real scenarios with them like give them a test case that whole family is invited on a dinner or birthday party. Tell them that they can be helpful by doing small things like placing cup and plates on the table or picking toys after playing.
Last but not the least; do not make this learning process a boring series of lectures rather do it in a natural manner. Treat them as your friends and discuss politely.