Are you returning the car to Germany? There is a big international drop fee to drop a car in another country. They cannot rent the car again until it has been transported back to the country where its license plates were issued, and you have to pay for that. Also, you will need to buy a Swiss vignette for the car when you enter Switzerland to use the motorways. Cost is 40 CHF.
Train could be pretty painless if you can handle your aunts luggage. Trains are very comfortable, but you will have to change trains a few times. You do not have to go all the way back to Munich. You can change trains at Buchloe, then at St Margrethen to get to Chur.