Sassette actually has the same origins as Smurfette (with a minor difference, she wasn't made by Gargamel). The smurflings (Slouchy, Nat, and Snappy) were pitying Smurfette who felt lonely, being the only girl in the village, so they decided to help her. They learned from Papa Smurf that Smurfette was indeed created magically by Gargamel out of clay. They then went to steal Gargamel's formula and magical recipes for creating Smurfette. They successfully created Sassette, who this time didn't need plastic smurfery (though her rude behavior and attitude needed a bit of chemical tweaking). As they introduced Sassette to Smurfette, Smurfette was overjoyed at having a female friend, albeit a bit of a tomboy.
Of course, they didn't realize that Gargamel had cast a spell on the claybank that would have quite literally turned the little smurfling into a living bomb when the noon sun struck her. Papa's quick thinking and a fortuitous cloud over the sun saved both Sassette and the other smurfs. Gargamel, getting too close to the clay, ended up a victim of his own treachery.
Bless wikipedia.