CAJUN ACCORDION DISCUSSION GROUP
Here's one that sold for $170 a couple of years ago. No idea how it compares to the one your mother-in-law has in terms of features, condition, age, etc. https://www.icollector.com/Early-Louis-Miller-San-Fran-CA-button-accordion_i32063547 There's a fair amount of information about the maker available on Melodeon.net, e.g.: http://forum.melodeon.net/index.php?topic=14968.0