Do capitol gains apply?

Do capitol gains apply?  My wife has joint tenancy on her mother's property.  Joint tenancy was entered in 2015 and a her mother passed away in 2015 after the joint tenancy agreement was put in place.  My wife lived in the property for a couple of months in 2015.  Can this property be designated principle residence for 2015?  If not would the adjusted cost base be calculated from the date my wife became a joint tenant in 2015 or when the property was originally purchased in 2001 (significant difference in property value)?