The Quevâtre family name originates in Normandie, France. There is a small village of the same name (although the spelling has changed here and there over the years). There are records at Mont St. Michel of a 'Baron de Quevastre' who was born in 1116. The Quevastre name is fairly rare with only a dozen entries in The French phone book. 

There are also records of a Pierre Quevastre who was born in Tourlaville (near Cherbourg) and died in Guernsey in 1808. Prior to that, the Quevâtre name did not exist in Guernsey. 

There is a partial ancestry tree here