I'm going to make a move on my friend's sister
What makes you friend's sister special in that you want to make a move on her?
She's really hot and she's single
It is understandable that you think your friend's sister is hot. If you like your friend's sister then you should ask your friend if it is okay.
It's fine. I want to make a move on my friend's sister.
I hope everything goes well for you. If it does not you should not make things weird or keep trying to pursue her.