Transport from Barcelona airport to Sitges

From the airport station take the R2 North train just for one stop, get off at El Prat De Llobregat.
At El Prat you change trains and take the R2 South to Sitges. It's only about 30 min, and there is a train every half hour. The tickets are cheap since they are Cercanias (commuters), for sure less than 5€. I've taken the R2 South train to Sitges from Barcelona Sants when my brother in law lived in BCN. Last time was 4-5 years ago. Nice little beach town.
The airport station is across the air bridge from T2. I think there is a bus from T1.
If you go by taxi or private car transfer, it's probably close to 90-100 euro.

You might want to check a bus service, straight from the airport to Sitges: http://www.monbus.cat/en/
Roberto's suggestion, while totally doable, might not be the best option -both time-wise and cost-wise- as you'd be taking a train towards Barcelona (opposite from Sitges) for a few miles to El Prat de Llobregat and then taking another train backwards to Sitges (http://bit.ly/1GG1mqN). By the way, note the commuting lines are called "Rodalies" which is also their brand name, not "Cercanías", that's only for commuting lines outside Catalonia.

Thank you! I found a Sitges cab company that picks up at the airport for 50 euros. But I think the bus might be the best option at this point. Thank you all.

Recommend the bus...get directions to the pickup spot at the information desk. It is downstairs/elevator, you are on the first (second) floor. The busses are well marked, but ask the driver to be sure. The stop in Sitges is just past the overhead bridge of the railroad. Almost at the center of town. You can easily walk to your hotel from it. Again, ask directions. I write out the address and show it so as not to be confusing to the one I am asking directions.

Thanks for the information. The bus seems to make a lot of sense both in terms of cost and efficiency. I checked out the website and I can see that our hotel is walking distance from the bus stop, so that's cool. Thanks for all the advice. We've been to Sitges a few times before, but we always took the train from Sants in Barcelona. This will be the first time going direct from the airport. Time to buy some sunscreen.