A host is someone who serves or boards a customer, it can be male or female. But hostess is female. http://www.dictionary.com/browse/hostesses

Yes it is a "verb" in this sentence. Sheldon is saying that his sister works at Fuddrukers (McDonalds), but this is not really the most important thing that he is trying to say in this statement.

It is a verb, but it should say 'hosts' instead of 'hostesses.'

First off, hostess is a noun. Hostess is the female version of Host (although you can still call a woman a host). In order to be a host, one has to be an individual or group of people, that hold and event for one or more people. ex. "thanks for having my wife and I over for dinner, you've been an excellent host" In this example, the people who invited the husband and wife, would be the hosts. ex.2 "we're throwing Bob a retirement party at Jerry's Paintball Stadium" In this example, Jerry's Paintball Stadium, is the host hope this helps