There's a possibility that we may be traveling to Europe with my sister's family, making it a total of 11 traveling together. Can groups that big get a discount on airfare? Would I have to go through a travel agent?
Ten or more is needed.
First, go to the airline's website and check out their webpage on group travel. They will spell out the benefits of group travel and give you a phone number to call. Call it and get a quote for what you want.
Or, you can go through a TA but they will charge you.
Remember, the more people between you and the supplier (the airline) the more it will cost. The airline prefers you deal directly with them.
And, check more than one airline.
And, if you're rally savvy, and say want to travel Airline A vs Airline B but B gives you a better deal, call Airline A and see if they'll match Airline B's quote.