Girlfriend is Extremely mad that I don't believe her sister's pathological liar boyfriend's stories?

I love my girlfriend dearly, however last night we got into an argument over something really dumb. My girlfriend's sister is currently dating a guy,who I believe to be a pathological liar. I work 6 days a week in my family business and try to do everything for my girlfriend, we have been openly talking about eventually getting married. Basically here is what happened... my girlfriend's sister is dating an unemployed guy who is a screenwriter. He currently lives in Florida and she lives in New York he sees her once every 4 months, Every week he calls her up and tells her that her is going to make millions of dollars and is having dinner with numerous HUGE celebrities every night( for example Paul Mccartney, Brad Pitt and Leonardo DiCaprio), he tells her that his screenplays are being picked up by major studios and is looking for a penthouse apartment in NYC, this is all coming from a 21 year old who doesn't even have his drivers license and has never held a job, nonetheless. He is always extremely vague about his stories, cannot prove anywhere he went and there is not one mention about him on the Internet..ANYWHERE. I work hard to give my girlfriend and I a great life, take her away on vacations and do whatever I can for her. For some reason, my girlfriend, her sister and her mother seem to believe that this kid is telling the truth. Last night my girlfriend started crying and saying that her sister is going to have a glamorous life one day and we are just going to be "comfortable", she also got mad at me that I doubt his talent. I am so confused over this argument, that I don't even know how to approach it. Help me!