The company PLANA operates a bus service from Barcelona airport to Sitges: https://busgarraf.cat/en/
The company website shows timetables and prices.
The bus stop is at Terminal 1 (T1). There is a free airport shuttle bus for passengers arriving at Terminal 2 (T2) located under the bridge outside T2B. The distance between the two terminals is 4km and the ride takes between 10-15 minutes. The shuttle bus runs every 7 minutes, 24/7.
Once you´ve arrived at Sitges, follow these instructions (9 minutes on foot) to get to Destino Sitges apartments.
The airport train station is situated opposite Terminal 2 (T2), and is connected to this terminal via an airbridge. There is also a Bus transit service between the train station and T1.
To get to Sitges you need to change trains at the station of El Prat de Llobregat and take a train in the opposite direction (VIA 3. VILANOVA I LA GELTRU direction)
Itinerary: Airport-El Prat de Llobregat (one stop). Service from the airport starts at 05.42 until 23.38. Trains run every 30 mins. Change trains at El Prat. Train for Sitges on Line R2 Sud takes 24 mins. Trains run every 30 minutes.
Once you´ve arrived at Sitges follow these
instructions. It’s 5 minutes on foot. Follow the Google Maps instructions: You must pass underneath the train tracks.
There are taxi ranks at the main exits for Terminal 1 (T1) and Terminal 2 (T2). The fare for the ride is shown on the meter. In addition, there are surcharges for luggage, port/airport pick-up and drop-off, and toll charges if you take the motorway. (Fares are shown on a price list inside the taxi).
Taxi service is available 24/7.