As you said in your post, that you don't have a sister, you may find the idea strangely hot as sexual imagination kicks in, but in reality you could maybe begin to answer your own questions if you picture a scenario with an actual female family member in the picture (aunt or mother, for example). Sheer reality makes the thought pretty gross to most of us, I'm guessing.
If you happen to be attracted to a member of your own family, you'd probably end up whacking off to them anyway. If you happen to find the idea revolting, porn or not, it would certainly be something you wouldn't do, and therefore try your best to avoid scenes with your family member in it (may end up being put off porn altogether).
I'm fairly sure Belladonna's younger brother used to hang out at some of her sets. Quite what he got out of it, I'll never know.
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