Is this a common problem with speech?

Question

Whenever my sister says the word "nephew" she pronounces it as "nethew" and when she says the word "deaf" she pronounces it as "death". Does she have a speech problem because she replaces -ph and -f sounding consonants with -th?

Answer 1

Yes.  She has a lisp.  She should see a speech therapist, as speech problems such as lisping or stuttering can be corrected.  

Answer 2

yes! she is having a pronunciation issue  

Answer 3

if your sister commits the same mistake all the time, she probably has a problem in pronouncing words like that properly