my cat keeps leting a kitten nurse

i have a female cat her names nikki well shes never even had kitten's we have had her since she was a baby, well my mom wanted to get another lil baby to add to the family her dad's cat had 3 kittens and when she made them stop nursing he gave us one. every since then nikki has started purduceing milk for the baby ( i even looked to see and she is ) the thing is he is almost as big as she is now and he keeps on nurseing she wont make him stop ... can it hurt her to keep leting him nurse untill we have him fixed ?

No, I dont think it is directly dangerous to her.   The professional milk producing nurses of old were too such ones...  So they were not always "just beeing moms themselves".

Still,  I think the best in the long run is to stop him nursing on her this way.  for example, set a body sock on her.  And give him more to do.  Play more with him, etc...

Fixing him wont stop him nurse.  So although you shall fix him, its no solution for THIS problem.

Good luck!