Are the sister chromatids exactly the same?
A new chromatid is synthesized by copying the other chromatid. As such two sister chromatids are (quasi) perfectly identical.

Why did I say quasi? Because there are sometimes "errors" when replicating DNA. These "errors" are called mutations. As far as I know, most mutations occur during meiosis though and only very few occur during mitosis. So, to a pretty good approximation, yes sister chromatids are identical.