A few avenues to consider. 1. Did your grandparents attend a synagogue? If so, you might find conversion records there or at a nearby synagogue that has/had a mikvah. 2. What do census records from the times say? 3. Does the Catholic cemetery have records that show religion at time of death?
FWIW, you, your sister and your mother can convert at any time. Also, Reform Judaism considers the child of a parent who is Jewish to be Jewish, whether it's the mother or the father.