  We study the single interval entanglement and relative entropies of conformal
descendants in 2d CFT. Descendants contain non-trivial entanglement, though the
entanglement entropy of the canonical primary in the free boson CFT contains no
additional entanglement compared to the vacuum, we show that the entanglement
entropy of the state created by its level one descendant is non-trivial and is
identical to that of the $U(1)$ current in this theory. We determine the first
sub-leading corrections to the short interval expansion of the entanglement
entropy of descendants in a general CFT from their four point function on the
n-sheeted plane. We show that these corrections are determined by multiplying
squares of appropriate dressing factors to the corresponding corrections of the
primary. Relative entropy between descendants of the same primary is
proportional to the square of the difference of their dressing factors. We
apply our results to a class of descendants of generalized free fields and
descendants of the vacuum and show that their dressing factors are universal.
