I'm going to ghost my buddy's older sister, is this going to haunt me later?
Why do you want to ghost her? Is there something about this you're not telling me?
I want to ghost her because she's negative all the time, seeks attention too much, and also because she has bad taste in men.
It's rude and wrong to ghost someone and especially so if that person is close to your own social circle. You're going to occasionally come in contact with your friend's sister if you hang around your friend often enough. You should try to be respectful and maintain the peace between you and your friend's sister for the sake of your friend.
I can ghost anyone because then I don't have to see them.
You have to know that your friend's sibling is also a part of him in some way. If you ghost her then he will be offended too. Then he won't be your friend, do you see that now?