@m3g4n – Frances2 has a very good idea, but why not forget the cables and just use the ham radios? Ham radio transceivers can broadcast and receive transmissions for over 50 miles, and some can let you talk with people from the other side of the planet. In my opinion, ham radio is the perfect solution for you.
For a small broadcasting radius (10 miles or fewer), tune all your radios to the 440 MHz band or anywhere from 144 to 148 MHz (each radio must be on the exact same frequency). On a really big piece of land, with no other houses nearby, those frequencies will let you and your sister communicate, without broadcasting your private conversations all over the place.
But, ham radio isn’t secure or private, so don’t broadcast any personal information over the airwaves, just in case.