the young children are petting a piglet
The siblings are petting their pet pig.
Not all children are siblings.
exists e1 s1 x1 x2.(n_child(x1) & a_young(s1) & Theme(s1,x1) & Theme(e1,x2) & Actor(e1,x1) & v_pet(e1) & n_piglet(x2))
exists e1 s1 x1 x2.(n_pig(x2) & a_pet(s1) & Theme(s1,x2) & of(x2,x1) & n_thing(x1) & n_sibling(x1) & Theme(e1,x2) & Actor(e1,x1) & v_pet(e1))