Not a parent, not friends with my adult sibling: I suspect having emotionally mature adults around them would help. Also, don’t constantly side with the child with easier needs when there is a conflict between children. (Example: I wanted to not hear my brother’s music in my room. Brother wanted to play his music. Brother got his way. I got ear plugs.)

Don’t make the older child always do the selfless thing because they’re “more mature”. They shouldn’t have to share everything. (Example: brother got to ‘help’ blow out the candles on my birthday cake. He spat all over it because he was a toddler.)