  The relativistic approach to electroweak properties of two-particle composite
systems developed in Ref. [A.F.Krutov, V.E.Troitsky, Phys.Rev.C. 65 (2002)
045501] is generalized here to the case of nonzero spin. In developed technique
the parametrization of matrix elements of electroweak current operators in
terms of form factors is a realization of the Wigner--Eckart theorem on the
Poincar\'e group and form factors are reduced matrix elements. The $\rho$ meson
charge form factor is calculated as an example.
