Both houses should get the homestead exemption, based on the information provided.
Your mom should get the exemption for herself on her old house (because it was her primary residence on January 1, 2019).
The seller should get the exemption on the new house (because it was their primary residence on January 1, 2019). Your mom is simply assuming the taxes owed by the old owner as part of the sales contract.
The only way it should get screwed up is if your mom or the seller didnt actually live in the appropriate house as of January 1 and the CAD figures that out, or if there were other partial owners that didnt live there, etc.
Your mom should get the exemption for herself on her old house (because it was her primary residence on January 1, 2019).
The seller should get the exemption on the new house (because it was their primary residence on January 1, 2019). Your mom is simply assuming the taxes owed by the old owner as part of the sales contract.
The only way it should get screwed up is if your mom or the seller didnt actually live in the appropriate house as of January 1 and the CAD figures that out, or if there were other partial owners that didnt live there, etc.