. . . lawful in what sense and to whom?
But the big question is: is it OK with the Church if we are buried in the family plot which is in a Methodist church cemetery.
Dave and Bonnie Bj —
In context, “lawful” simply means that a law of the Church that obliges a person or persons has been observed and not violated, or that an obligation to observe a certain law was dispensed from by a competent authority of the Church. A law can be a universal one that binds all Catholics, or a particular law, for example, one that binds only those Catholics belonging to a certain diocese.
The dead are not subject to the jurisdiction of the Church anymore, so a particular law regarding the place of interment would bind those persons making the funeral arrangements and only if they were Catholic (since ecclesiastical law only binds Catholics according to canon 11). We can safely assume it would also bind the cleric or authorized “lay minister” conducting the rites of committal, etc.
There is no universal law regulating the place of burial. It reads that “everyone, unless prohibited by law, is permitted to choose a particular cemetery for burial” (c. 1180 §2).
But a diocesan bishop could establish a particular law about this, which would bind the Catholics of his diocese (c. 12). (There are some variations about how particular law obliges travelers in a diocese or binds those subject to a diocesan bishop but outside of his diocese, but these are probably not relevant to this situation.)
However, if there were such a particular law, either the diocesan bishop or a local ordinary (for example, the vicar general) could dispense from it (cc. 87§1 and 88) whenever this relaxation of the law would contribute to the spiritual good of the faithful.
I would tend to think that a grave in a non Catholic cemetery would not have been blessed already, but the Prayer over the Place of Committal contains options for such a blessing.
I would be surprised if there were a particular law governing and prohibiting interment in a non Catholic cemetery. But you can and should check with the parish priest about such a law, if any, and its scope. However, I would also think it likely that the diocesan bishop or local ordinary would be inclined to grant a dispensation under the circumstances.