It sounds as if your sister has a dual fuel system, electric HP with furnace backup. What is fuel source of furnace, nat gas or propane?

And yes, you can pair a heat pump with conventional furnaces as well as high efficient models.

IMO

They have NG. She said at first she thought they had elec hot water. Now it seems they have NG. They do have outside finned condenser. So that's airsource? They have heated garage floor too.