The Smithin family can be traced to Pershore around 1600. It is a rare surname and they are all part of one family.
Before 1600 the family name was Smithsend and we have details of the Smithsends from about 1400. They lived around Tewekesbury and several branches carried on with surname Smithsend but with the Smithins it seems to have gradually changed from Smithsend to Smithin.
There are two main branches; those living in Birlingham and those who lived in Stoutlton. There are modern descendants of both these branches.
Smithin family in Birlingham
Smithin family in Stoulton