This may be simply a cosmetic issue. She may be sneezing to clear her nares which are clogged with dirt and maybe food. It's not easy to clean inside the nares, but you may need to try.
One nare goes all the way through to the other one. You can insert a very skinny syringe filled with saline and flush them. Lay the chicken on her side and flush one nare. It should drain out the other one. Flip her to her other side and repeat for the opposite nare. You may need to find something to dig out the gunk inside the nares if flushing doesn't work at first.
One nare goes all the way through to the other one. You can insert a very skinny syringe filled with saline and flush them. Lay the chicken on her side and flush one nare. It should drain out the other one. Flip her to her other side and repeat for the opposite nare. You may need to find something to dig out the gunk inside the nares if flushing doesn't work at first.