I Was Shocked

I overheard my dad talking to my mom. He told her that if my sister (who's Jewish) married a Christian, he would basically shun her. I think he means it. He's very serious about us marrying within the Jewish religion. He's worried if we'll have kids, we won't raise them Jewish. I knew he felt strongly about marrying Jews but I didn't think he would go so far as to say we'll be dead to him if we do. I see things from his perspective in which a lot of religious people feel the same way. But he should also be thinking about [i]our[/i] happiness. This guy my sister is seeing makes her happy. When my mom wanted to talk to him more about it, he had a tantrum and went into his room saying he doesn't want to talk about it anymore and slammed the door like a child. I'm not saying they'll last in the long run but they both deeply care about each other. He goes and says that my mom is "encouraging" this. All she's encouraging is my sister's happiness. The only thing that would disappoint my mom is if we converted out of the religion which we told her will never happen. What's your opinion on this?