The bodyshell of your car is a "structual component "... rust holes in the doors, wings, bonnet and boot lid are not failiure items, as long as they are secure, and wont injure anyone with sharp edges, however, if you have corrosion on your front wing, lets say, and its within 30cms of the suspension top mounting, then its a failiure.
Bolt on Xmembers that have corrosion, are best off being replaced rather than repaired.
To anwser your question, yes your sills are stuctual, and they will need welding up!
Tom