if u have secks with ur sis. this will happen:
good fuck, now you are starting to get paranoid... blablabla, you cant look her in the eyes, cant look ur foster parents in the eye, fosterparents will sense that something is wrong, they will prbly find out sooner or later. Then they'll kill you with an axe:P
or even worse, she gets pregnant with a child, and dosent find out until its to late for an abort blabla you are stuck with a child for the rest of your life.