Tenants in Common

Their Basis would be Mother's basis, if Mother deeded the house to them. Which may be what she originally paid for the house, (Plus Improvements) or if she was married, possibly received a step up Basis if spouse passed away while they owned the house jointly.

Unfortunately they don't qualify for the personal residence exclusion.

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.

It's fairly obvious the mother's name was not on the deed, if two people received half the proceeds each.  The issue is whether the mother was still beneficial owner of the house, even though she did not have legal title.  The sisters might want to make the case that they held it only as constructive trustees, and the sale should be reported by the mother, who may qualify for the primary-residence exclusion.  Having gone to great effort to avoid probate, it now depends on how much effort they want to make to avoid the tax collector.  

Why were they owners of the mother's house?  Did the mother own the home ,then transfer title to them, but the mother kept full control and responsibility of the house?  If so, that may be an implied Life Estate, which would require different treatment.