Here's what I would have said to Katie's dad if I were her sister

If I were her sister and while Katie desperately admitted to her mom that she was a victim of domestic violence and her mom convinced her to leave Jimmy, I would have called her dad at work and immediately told him about Jimmy's abuse towards Katie and then he would have shaked and hung up with me and then he would have gone to Katie and Jimmy's house with the other cops to arrest Jimmy, revealing that he knows about the abuse and when he denied it, he would have angrily read him his rights. And here's what the dialogue for it would have been then;

Sharon Liner: Hello, Dad?

Jack Liner: Hey, Sharon! What's up?

Sharon Liner: Dad, there's something I gotta to tell you and I'm sorry I didn't tell you earlier but Jimmy's not who he says he is. He beats Katie every single day. That's why she had that black eye and she's got bruises all up and down her back too. I saw them when I stopped by her place before going back to school while you and Jimmy were out duck hunting and she just didn't want me to tell you and I'm really sorry, Dad. I should have told you.

Jack Liner: OK, OK, Sharon, you did the right thing. I'll talk to you later.

At Katie and Jimmy's house after Doug and Wade drop Jimmy off and Jack shows up with the rest of the police...

Jimmy Pettit: Jack, what are you doing here?

Jack Liner: Jimmy Pettit, I have a warrant for your arrest.

Jimmy Pettit: For what, Jack?

Jack Liner: My eldest daughter, Sharon has informed me about the nature of your marriage to my youngest daughter, Katie.

Jimmy Pettit: But Jack, we're family. I love Katie and Katie loves me.

Jack Liner: So, you never beat her?

Jimmy Pettit: No!

Jack Liner: You never gave her that black eye? You never covered her in any bruises?

Jimmy Pettit: If that's what Sharon is saying, then she's lying! I would never hurt Katie -

Jack Liner: Don't you dare twist this around on Sharon! Don't do it! You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to an attorney. If you cannot afford an attorney, one will be provided for you. Do you understand the rights I have just read to you? With these rights in mind, do you wish to speak to me?