  The relativistic approach to electroweak properties of two-particle composite
systems developed in previous work is generalized here to the case of nonzero
spin. This approach is based on the use of the instant form of relativistic
Hamiltonian dynamics. The generalization makes use of a special mathematical
technique for the parametrization of matrix elements of electroweak current
operators in terms of form factors. In this technique the parametrization is a
realization of theWigner--Eckart theorem on the Poincar\'e group and form
factors are reduced matrix elements. As in the case of zero spin the
electroweak current matrix element satisfies the relativistic covariance
conditions and in the case of electromagnetic current it also automatically
satisfies the conservation law. Physical approximations such as, for example,
the relativistic impulse approximation, are formulated in terms of reduced
matrix elements. The electromagnetic structure of $\rho$ meson is calculated as
an example of realization of the technique proposed.
