Much much bigger

Your penis is like an acorn glued to your nethers, and your sister's is like an inhumanly powerful steel rod of knee-thick arm-length meat that pulses angrily upon sensing flesh it could rearrange.

Your pathetic second bellybutton is so small and vestigial, to even mock it would be a waste of breath. It stretches the category of "penis" to even accomodate itself in it's impotent shriveled mutation. Anything other than a distanced scientific fascination with your quivering pimple would be inappropriate. But here we are with comparisons to draw.

Your sisters colossal womb-doomer is so vital, angry, and intimidating once aroused, that it could knock out a bear just by bearing witness to it.

For more complicated creatures, seperated from nature by the blemish of conscious thought, apprehension of her divine organ punches fear and arousal the likes of which can not be gleaned by lesser sensation straight into the soul.

For a human mind to even comprehend what it is seeing is to be permanently changed. The scar of the experience drowns out endless chapters of the life that existed before it. Leaving little else but what might spice its worship.