18 To obviate invalid marriages when Eastern Catholics marry baptized Eastern non-Catholics and in order to promote fidelity in and the sanctity of marriage, as well as peace within the family, the Sacred Council determines that the canonical "form" for the celebration of these marriages is of obligation only for liceity; for their validity the presence of a sacred minister is sufficient, provided that other prescriptions of law are observed.
Source: Orientalium Ecclesiarum (Vatican.va)