You gotta watch this.  Timeless, beautiful harmony from Dolly, Linda and Emmylou from about 1976. 

And if that’s not enough, watch this one from 1986.  Listen really close and when Willie introduces them, the first little holler you hear in the audience is your Sista.  It was the CMA awards and I was sitting in the middle of the first row of the balcony, about to wet my britches cause I was so excited about this.  This was a few months before the Trio album was released.  Then, when they finish, I’m the first holler again.  Hee.  I was 17!