There needs to be more details provided.

Were they TIC with Mom also on the deed? Or, did they own it together and only the two of them? And how did they get it: they bought it for Mom, or one of them used to live there, or as noted, perhaps it was Mom & Dad's until Dad died, then Mom put only them on the deed (gifted the house) or Added them to her deed?

Every version makes something different happen.