Is it okay if I call my sister in law pretty?
I'm not sure. Why do you think it might not be okay?
I don't know if it's an appropriate thing to say.
I think it would be okay to call your sister in law pretty. As long as you're saying it in a polite way and not trying to make it weird, then I don't see a problem.
Yeah, I think you're right.
Probably, because in general people like to be complimented, and saying someone is pretty isn't out of line. I think it's fine.