https://en.wikipedia.org/wiki/Aircraft_pilot

I would like to know what decision do the commercial airlines carrier take when there is emergency case of a Pilot who cannot fly the plane because of personal emergency conditions?

Emergency conditions could be Pilot falling sick with a medical emergency, Family member, close relative death etc

The event has happened just few hours before the final plane departure.

Do the Co-pilot fly the plane in that case?

Do the specific Airline carriers always have Pilots as reserves ?