I agree with pixiepie, when it comes to such youngsters, its usually best to speak to your health visitor.
Saying that though...........does your neice only ever get offered mushed foods? Your sister could try offering her something like a rusk of a toast soldier. Even though she hasnt got the teeth to chew, sometimes they still enjoy have a good old suck! Also chomping on something hard like a rusk will help her with the teething. Also with that in mind, try biccie pegs, theyre designed with teething in mind.
Good luck :)