When they're little, the eldest one will love the little one to distraction - it'd lovely to see.
When they're toddlers, they all play together and have great fun
When they're school age, they all go to watch each other in school plays, concerts etc and say loudly 'that's my brother/sister!'
When they're teenagers, the big ones can help the little ones with maths that you can't remember how to do.
There's lots of contact with their siblings friends. - girls get used to boys being around and vice versa. Then when they have a girlfriend of their own, they understand them a bit better
On holidays and family days the dynamic is constantly shifting - different permutations of who wants to do what and who enjoys what. Two is just too symmetrical for me. And there's generally at least one of them who wants to do what I want to do!
Then when they grow up and go abroad for their gap year And they come back, it takes a whole week to share everyone's stories for the last year.
And they visit each other in either a two or a three and have their own relationships ( with their siblings) - richer for them as well as for we parents
Not yet at the stage of grandchildren but for my children, if they have the same kind of opportunities for getting together as I had with my brothers and their kids, family holidays with lots of cousins are brilliant