A schwannoma is a benign, or non-cancerous, tumor that affects the peripheral nerves, or the nerves towards the outside of the body, like in the arms, legs, and so on.
Because the tumor is benign and usually very slow growing, your sister’s doctor may have good reasons for wanting to wait; they could range from availability to perform the surgery to your sister’s health and whether she can have surgery. Only her doctor can explain his approach to her treatment.
If your sister isn’t satisfied, or would like to see if someone may operate sooner, she would be advised to get a second opinion.