Need help with my little brother?

My little brother and sister just moved in with my fiance and I. He's 17 and my sister is 14. My sister is great, it's my brother. He's been acting out a lot lately, my mom became sick, so I took them in after Christmas. Our father died years ago, so it was only me. I love my brother but he never listens, I let him have a curfew of 12, when his license states for him to be home, but he never comes home on time. I try and talk to him and he gets aggressive I guess would be the word. My fiance is at her wits end with him, he always calls her profanities. I just don't know what to do because he's so difficult.