This is a difficult question to answer and depends on many specifics of her diagnosis, staging, recurrence and other issues. In some instances radiation is used first to shrink a tumor and reduce the margins prior to surgery. The risk in this is that radiation slows and impedes the healing process.

The other scenario is prophylactic radiation done post surgery and that can damage the healing process.

It's a difficult treatment no matter what order.

There should be a meeting of the minds between the radiation oncologist and the ENT based on the best outcome for your sister. Tongue cancers tend to be more aggressive so whatever is the most conservative approach is what I what do. I would probably throw chemo at it too if it were me.