The phrase "Sweet Sister" is an expression used to address a female sibling in a loving and affectionate manner. It implies that the speaker considers their sister to be kind, gentle, and caring - much like something sweet or sugary.
I absolutely love these family photos that my friends sweet sister Norma shot for us. (outfitsandoutings.com)
Beryl Reid wolfs down the scenery as June, an actor known and loved as sweet Sister George in a TV soap but in real life a boozed - up monster given to molesting nuns in taxi cabs. (timeout.com)