If my dad buys my brothers ticket (hes 15) can he watch the R rated movie with me (I'm 18)

Yea

Yes

Actually no. If the staff know your dad is not going to stay for the movie, they won't sell you the ticket. A minor under 17 must be accompanied by someone 21+ for the entire duration of the R movie.

The key wording that applies to you is "accompanied by an adult". In your case you would need to show your ID to prove that you are technically the adult.