". . . okay?" As in, "We're going to go to the doctor's now, okay?" You don't have to clear every little detail with your child. And what are you going to do if he says "No!" as he probably will? Instead, give him the info he needs: "We're going to the doctor's in five minutes. Let's get ready."
"That's not nice." "Nice" is a weak word and doesn't mean much to kids. Think about what you're really trying to say. Hitting his brother is not allowed? The specificity will get him to stop -- and maybe keep him from starting next time.
"Can't you see??" Actually, no. Your kid probably can't see that you're checking your e-mail/talking to Dad/draining the pasta. They're a little self-centered, kids -- and that's normal. So fill your child in ("I'm talking to Daddy right now") and be constructive ("so you have to say 'Excuse me' or wait until we're done").