A boy and a girl playing on the beach.
A brother and sister play in the sand.
A boy and a girl aren't necessarily brother and sister.
exists e1 s1 x1 x2 x3 x4.(n_beach(x4) & a_topic(s1) & subset_of(x2,s1) & on(e1,x4) & Actor(e1,x3) & v_play(e1) & (x2 = x3) & n_girl(x2) & subset_of(x1,s1) & n_boy(x1))
exists x1 x2 x3 x4.(n_sand(x4) & a_topic(x1) & in(x1,x4) & n_play(x1) & of(x1,x3) & n_sister(x3) & of(x1,x2) & n_brother(x2))