Sneezing is normally an indication that *something* is irritating their nose, whether internal or external.
Cat's noses are very sensitive to things our big ol' honkers can ignore. Dust, perfumes/colognes, air fresheners, pesticides, humidity changes, etc. etc...
Does she sneeze more often in certain rooms or around certain objects? Has *anything* new been brought in her house? Does she go outside?
Are there any other symptoms beside sneezing? Watery eyes, over or under dilated pupils, runny nose, cracked or overly dry nose, excessive scratching, cough, "face rolling" (rubbing face into the carpet or pillows)? (All animals face roll occasionally but if she does it frequently there's usually a good cat reason.)
Best way to help the sneezing is to track down the irritant.