Let A be an abelian variety over a number field F , and suppose that Z[ζn] embeds in End F A, for some root of unity ζn of order n = 3 m . Assuming that the Galois action on the finite group A[1 − ζn] is sufficiently reducible, we bound the average rank of the Mordell-Weil groups A d (F ), as A d varies through the family of µ2n-twists of A. Combining this with the recently proved uniform Mordell-Lang conjecture, we prove near-uniform bounds for the number of rational points in twist families of bicyclic trigonal curves y 3 = f (x 2 ), as well as in twist families of theta divisors of cyclic trigonal curves y 3 = f (x). Our main technical result is the determination of the average size of a 3-isogeny Selmer group in a family of µ2n-twists.