The best way to teach your youngsters is by example. By helping others yourself, you teach them how to help others. Whenever your eldest does something helpful for you or anyone else, praise them and let them know how much you appreciate their actions.
Another way to encourage your older children is to let them know how good they can make their baby siblings feel. Let them hold the baby (with your supervision), and when they smiles at each other, you can say, “Look how happy you made him!” This will help the older ones feel like a vital part of the baby’s life. If your younger child is a toddler, encourage your eldest to show him how to do things, such as putting toys away or getting dressed, and suggest that they read to the little brother (which will make them both feel proud and happy).
Let them know they have a lot to offer the sibling, and that you’re proud of the example they’re setting.