The Scottish heroine loves an English lord above all Scots. Her family, learning of her love and (in most versions) her pregnancy, prepare to burn her. She sends tokens to her love, but she has been burnt before he can arrive. (He takes bitter vengeance)
Bronson, Roud, and Scarborough, and probably others, have filed "Mother, Mother, Make My Bed" with "Lady Maisry," but that ballad (composed largely of floating elements) lacks key plot elements, notably the reasons for, and fact of, the girl's condemnation and death. It appears to be a separate song, though perhaps composed on the fragments of this song.
Interestingly, it appears that every text Bronson has of "Mother, Mother" is part of his "C" tune group, and every text in the "C" group is either "Mother, Mother" or is too short to allow identification. - RBW