There are trains after 10pm from the Airport to Sitges (You have to make a change of trains.)
Monbus goes from T1 (you can get a free shuttle bus from T2.
Monbus has its own website.
The stop you want is Parc Can Robert (from the Airport it is the first stop in Sitges) - It is only a short walk from there to your Hotel.
There are online Maps that will show you both your Hotel and the Monbus Stop by the Viaduct.
The Monbus is from T1, the train is T2. In T1 at ground level go to bus bay 7 although the bus could stop at 8,9 or 10 if 7 is already occupied. The bus you want will have Monbus in big letters on side of the bus. Check with the driver who will get out to open the sides to put your cases in that they're going to Sitges and not going the opposite way to Barcelona. Pay the driver €7 each. The drive to Sitges takes about 30 minutes and Sitges is the first stop. You then have an easy 5 minute walk to the hotel