contains is not a standard matlab function. You can test set membership wih ismember. it'll return a vector of [true, false] values, so if you just want a scalar, yo need to use all.
Similarly, == returns a vector of [true, false] and only works if both matrices / vectors being compared are the same size. To test that two (sorted) sets are identical you'd use |isequal
Thus,
if all(ismember(neighbour_n{j}, set_n{k})) && isequal(set_n{j}, set_n{k})
%do something
