I hope you realize you let yourself be manipulated and is now a part of an abusive relationship.
First time? yeah right. Based on your initial post, your sister's and his reaction, its definitely not the first time.
I have a friend who hit his girlfriend. I knew him for many years, longer than your sister has known her fiancé. When I asked him WTF happened, he said he didn't know what came over him, he blamed it on alcohol and acted like he was shocked too.
After talking to his girlfriend(who was also my friend due to their relationship)and letting her confide in me, and asking heartfelt questions, I learned that this was not the first time. It was the 3rd or 4th. The first time was a push/shove, and this time was full on hitting. Each incident was worse than the one before.
You know what shocked me the most and angered me? The girlfriend forgave him and acted fine and continued to go out with my friend. I was speechless. I couldn't understand and I had to go to google to learn this mentality.
It was an eye opener. The girlfriend was a victim of an abusive relationship and manipulation, and my friend was an abuser and manipulator. Things he did and said were calculated. It made me realize that some of the 'off' things he said or did that I passed off as jokes because he was my friend were signs of a problem. This clarity applied to other people's behaviour as well. I was blind because I laughed things off based on the fact that they were friends/acquaintances but if I let myself analyze it, it was so obvious of an underlying problem. Depending on the person and the value of their relationship with me, I called them out on it or just cut off contact but of the few that was worthy of my attention, 100% had a personality issue.
And of those, 100% of them lied initially and continued to lie about it. But I let them know I knew. They knew I would ruin their social lives so hard if they didn't keep their problems in check. Friend says something questionable that I know has a fucked up thought process? I give them a quick stare. I have psychopathic tendencies and am also a master at manipulation. Could you imagine a person knowing your deep intentions dark or good, and having them communicate to you that they know by receiving a well formulated eye contact/body language? That's me. Thankfully I try to use it for good since my psychopathic tendencies are not full-blown and I still have empathetic and sympathetic traits. Yes its a superpower and thinking it of as a superpower is also a trait of psychopathy. I can't help it. Anyways,
Do you actually believe someone assaults their 5 year long SO and her sister for the first time based on the fact that he had water poured on his stomach while sleeping? Really? Wake up. (no pun intended).
It's also a known fact that therapists have a hard time with abusers because they are good at manipulating and learning. Therapy is a good way to learn how to be a better at manipulation. I have first hand experience.
You think a pastor can help with the words of jesus and love when a decade trained therapists have hard time?
5+ year long relationship that started at 17 and is now 'caught' as an abusive one? RUN.
You were the first witness to the abuse, it wasn't the first abuse though.