S.H.
Its all about "expectations'..... and whether or not it is age-appropriate... and then how it changes, just because he is now an "elder" sibling.
Keep in mind... an "elder" sibling... gets a TON of stuff and expectations put onto their tiny little shoulders... and they may not, per their age and development and maturity, be able to handle it.
The 'expectations' upon the Elder sibling... may simply be mis-aligned and too much.
Just because there are 'expectations' upon a child... it does not mean they 'can' do it, nor succinctly or accurately.... and they only get frustrated by it... hence acting out, good or bad.
A child, does not automatically become 'perfect' just because he is an Elder sibling... in fact, they need more... more understanding... and adjustment to the whole relationship and household changes and the changes in his Mommy and parents. Too.
That is a TON of 'changes' for a child.... lots of Elder kids, get 'stress'.... and this is manifested in different ways.
My daughter, who is the Eldest.... has even told me "Mommy, I'm just a KID! I can't be everything!" Even though, I am acutely aware, of her feelings being an older sibling. And how that impacts HER. She is expressive to us and we talk about it..... always.
And I never "expect" her to be the "example" for our younger child... because, she is only a child herself. And I never make her, give up or share everything with her little brother.... her things, is "hers." And she has to know that we understand her boundaries too etc. As well as, if something is taken away from her from her little brother... we stand up for her... and teach our younger child that he can't have everything... etc. and about manners etc.
Mostly, we teach our kids about being a "family" and how we have each other's back.... and that we all are on the SAME "Team."
Emotions.... are not even fully developed yet, in a 3 year old.
all the best, just some ideas,
Susan