We investigate the structure of the D-s1(2460, 2536)(J(P) = 1(+)) mesons via analyzing the semileptonic B-c -> D(s1)l(+)l(-), l = tau, mu, e and B-c -> D-s1 nu(nu) over bar transitions in the framework of the three-point QCD sum rules. We consider the D-s1 meson in two ways, the pure vertical bar c (s) over bar > state and then as a mixture of two vertical bar P-3(1)> and vertical bar P-1(1)> states. Such types of rare transitions take place at loop level by electroweak penguin and weak box diagrams in the standard model via the flavor changing neutral current transition of b -> s. The relevant form factors are calculated taking into account the gluon condensate contributions. These form factors are numerically obtained for vertical bar c (s) over bar > case and plotted in terms of the unknown mixing angle theta(s), when the D-s1 meson is considered as a mixture of two vertical bar P-3(1)> and vertical bar P-1(1)> states. The obtained results for the form factors are used to evaluate the decay rates and branching ratios. Any future experimental measurement on these form factors as well as decay rates and branching fractions and their comparison with the obtained results in the present work can give considerable information about the structure of this meson and the mixing angle theta(s).