logic - Prolog backtracks and executes other statements too -

assume have following facts:

sister(susan, jane). sister(karren,holtby). parent_of(susan,karren). parent_of(susan,holtby). 

and following definitions:

sibling(x, y) :-       sister(x, y).  sibling(x, y) :-       parent_of(z, x), parent_of(z, y). 

is there reason why goal sibling(susan, jane) gives true false. both definitions used?

is there way check definitions true , output true if so?

prolog tree based language. let's establish knowledge base

male(mark). female(mary). female(beth). brother(mark, mary). %mark brother of mary brother(mark, beth).  %define sister sister(x,y) :-   female(x),   male(y),   brother(y,x). 

in case clause sister evaluated such:

• bind mary x
• bind mark y
• check knowledge base brother(mark, mary)
• return true if found
• bind beth x
• bind mark y
• check knowledge base brother(mark, beth)
• return true if found
• no other bindings exist, return false

in prolog possibilities tried. return true cases until can't find one, automatically return false (i.e. nothing can satisfy this). strength , confusion prolog.