If it about attention, remember the older child has been pushed off his pedestal by the younger one. On the other hand, the younger one is not yet as independent as the older one. They all need attention. Hug them, tell them you love them, tell them they share something very special, you! Help them know that you will always give them attention but like a box of cookies, they have to share you.
If it is about frustration, teach children to take a deep breath and to take a break if things are not going their way. Help the children change activities, declare 10 minutes of peace where everyone does an activity on their own, even you!
Expectations, everyone has them. You might have a family meeting to discuss what the children expect of each other, what you expect of them, and what they expect of you. Make a list.
In our family we expect:
To work out our differences peacefully.
To love each other even when we are arguing.
To ask permission before taking someone else's belongings.
To play fairly.
To be kind
You can make the list as long as you want, but remember it all boils down to
Siblings always know how to push each other's buttons. Believe it or not the majority of children learn to love their siblings. They love you, they share you, they want to make you proud. So they share this goal, they share laughter and love. If they can share these things, they can learn to share everything.