How do I get my dad to understand that I don't want him to date or marry my best friend?

I have known her for years. She was dating my brother-in-law when we met. My kids think of her as their aunt. On Halloween 2014, I lost my mom to cancer. My mom and dad were still married when she passed away. My friend was there for me through that and my own cancer diagnosis. She has been a very big part of both me and my kids' life, but now last month, my dad told me that he really likes my friend and wants to marry her. She's like a sister to me. My kids hate the idea.