The sign for "FAT" (as in "a fat person) uses "claw" hands near
the face to show chubby cheeks. Start with a relatively normal face and then
puff out your cheeks as you move your hands out a couple inches.
Is your sister fat?
If you mean fat as in the marbling on a steak just fingerspell F-A-T.
You can indicate relative degrees of fatness by how far out you move your hands and
how much you puff out your cheeks.
You can also show specific aspects of being fat. For
example suppose someone is pot bellied.