I think you have to specify "mother-in-law" from the wife's side or from the husband's side, because, for example, in Chinese literature, these two different "mothers-in-law" are portrayed very differently, almost in total opposition. My guess is in English literature they are also not treated alike. 

Hi Matthew,

Thanks for the comment... yes, our research is focused on the mother-in-law from the wife's perspective, that is the representation of the husband's mother for the wife. Most of the time there seems to be a rather negative notion of a threatening and invasive/competitive presence for the wife but we were wondering if this is the only option, can't there be some positive images???

Your topic is very impressive. Try Chaucer's The Canterbury Tales: The Man of Law's Tale. Try also some English translations of the Arabian Nights. Good luck.