He jerks in front of his mother and sister, they end up watching him and ...

She was masturbating indoors while watching her sister and mother through the glass, while they are in a bikini sunbathing. There comes a time that the sister sees him, but says nothing, he watches. Finally he leaves his mother without saying anything and goes to help his brother with the straw, helps him so well that he gets to run, the surprise comes to an end when the mother surprises them both!