Lots of questions will come from the posters here - asking for more information, so expect that.
Firstly, I would strongly doubt that he would only be in hospital for 3 days following a total glossectomy. And I would expect that he will require some method of being fed while he learns to eat without a tongue - and yes it can be done.
Alot of what will happen post surgery will depend on the pathology from the surgery, so it is impossible at this point to say what all will be required.
It took a year for me to be diagnosed correctly, and my story is not unusual. Oral cancer is often dismissed, misdiagnosed, diagnosed as something else, etc, etc, so the time frame that you are talking about from first symptom to surgery seems very short, which is a good thing - assuming he wasn't being stoic before that.
Stay close, ask questions, see if you can pin your sister down for more concrete info that way the people on the boards can be more help to you.
Take a deep breath its just starting. Sounds like you will be a wonderful support for your sister and her husband, and it sounds like you are a wonderful person to so actively be involved.
Donna