Just a guess, I'm hoping someone else here can confirm re: bobbin case. Class 15 bobbin cases generally come in two styles,. The difference is which way the positioning finger at the top points, either to 11:00 or to 1:00.
Take a look at the bobbin case in your Brother machine (assuming the Brother does not have a drop in bobbin case.) Does it fit the inherited machine?
Take a look at the old machine - there should be a "finger receptacle" where the bobbin case fits. Does it point to 11:00 or to 1:00?