How do we intervene with this strange and slightly creepy relationship?

[hollowSamurai986]

My sister is sixteen years old. Our father's best friend is twenty-three years old.

After seven years of oblivion, my mother and I have figured out they fancy each other. Now, they have never done a single thing about it - it's so far just a mental thing, which is good,right?? But they do talk a lot on facebook, and text each other constantly. They do not know we know.

How can we tell them we know, and intervene effectively without destroying relationships within the family? Please help!