There has to be a disruption. IMO, using "sister airport", "area airport" and "co-terminal" interchangeably is misleading.
"When a particular airport is affected, and Southwest serves a "sister airport" or "co-terminal", we will offer the flexibility to depart from/arrive into an alternate airport. For example, if San Francisco International Airport is affected, Customers may be offered the opportunity to change to the San Jose or Oakland airports."