The terms YAM & Sweet Potato are US marketing ploys.
'Yam' is the Soft version---orange flesh
'Sweet Potato' is the Firm Version
YAM and Sweet Potato have NOTHING in to do with Potatoes ----different plants/different tubers
Sweet Potatoes are related to Morning Glories
TRUE YAMS are very difficult to find and are imported ---can be found in 'ethic' grocery stores
True YAMS are related to palms and grasses
SO....
If you want to serve Potatoes and Sweet Potatoes together go ahead.
If you want to serve Sweet Potatoes instead of Potatoes go ahead.
OR Potatoes instead of Sweet Potatoes go ahead
But they ARE different so Hubby's MOM is correct