  The relativistic approach to electroweak properties of two-particle composite
systems developed previously is generalized here to the case of nonzero spin.
This approach is based on the instant form of relativistic Hamiltonian
dynamics. A special mathematical technique is used for the parametrization of
matrix elements of electroweak current operators in terms of form factors. The
parametrization is a realization of the generalized Wigner--Eckart theorem on
the Poincar\'e group, form factors are corresponding reduced matrix elements
and they have the sense of distributions (generalized functions). The
electroweak current matrix element satisfies the relativistic covariance
conditions and in the case of electromagnetic current it also automatically
satisfies the conservation law.
