All of these

Of course it's all of these things. Did you forget the difference in volume between yours and your sister's penises again?

It's one thing to forget your penis is smol, but that your sister's is big? Unforgivable.

That thing is slapping her ankles raw as she walks around the house in oversized t-shirts all the time.
She's constantly sneaking into your bed for late night cuddles and resting that trouser leviathan across your entire torso and face while she sleeps.
The bathtub is filled to the brim with her dense steaming oobleck every god damned day.

If there's a big dick related thing that can happen in this house, it does. And it does because your sister has one. A really really really big ONE. A dick that is. Her dick is big. It's a big dick. That she has and you don't. Get cucked. (oh wait, not yet)