Katherine and Patrick Schwarzenegger
08/23/2011 at 10:00 AM EDT
A shirtless Patrick Schwarzenegger
spent a playful day on the beach in Malibu, Calif.
And he wasn't alone.
Nearby, sisters Katherine, 21, and Christina, 20, enjoyed a swing, and after a dip in the water, Katherine teased 17-year-old Patrick's wet hair into a Mohawk.
Youngest brother Christopher, 13, also appeared happy and healthy after a surfing accident
last month on a nearby beach.
The siblings have spent part of the summer vacationing with their mother
, Maria Shriver, in Cape Cod, Mass. – Liz Raftery