Are these sentences possible?
I am Jane and I am 17. My ELDEST brother Tony is 20 and my YOUNGEST brother Jack is 16.
Could I substitute eldest and youngest with big and little?
What if Jack was older than Jane?
In which case should she say younger instead of youngest in this context?
Big and little are fine. When you have three things/people/objects to compare; take yours for example, one of them is the youngest and one of them is the eldest.
Jack is younger than Jane.
Jack is the youngest (of all).
You can use comparative concept to express a superlative idea.
Jack is younger than any other one.
I hope that helps.