I have a turkey sitting and I was worried about the same thing. At first she was allowing two different hens to lay their eggs in with her and then she would pull the eggs under her. Now she won't let anyone or anything near her nest...I have the peck marks to prove it!
Now I just worry about her being able to keep her poults safe when they hatch.
Given the beating she has given me for removing the unfertilized chicken eggs..I suspect she will be able to defend her babies. I have a brooder ready just in case, and I have the ability to separet them if I need to. I'd really like her to just mother them as nature intended. My stroke of luck may be that except for the two laying hens, everyone else is either teenagers or younger. I think they will respect Momma turkeys wishes.