Why does rape of sex actions happen between sibilings?

It’s so scary and crazy to think theres real life situationswhere this happens all the time and sometimes as a parent or adult you never notice? Why do sibilings have to rape their own brother and sister? It’s such a scary thought. I currently have a daughter of 1yr 6MO and pregnant with our second 36Weeks having a boy. Its scary to think this. How can we avoid this stuff? How can we talk to our kids if sometimes they don’t want to speak up?