Complex sentence: Phyllis and her sisters had mimicked the singing style of the Andrews Sisters, the Mills Brothers and the Dinning Sisters since the were young, when they would perform short shows for family and friends in their parents' living room.
Split into simpler sentences: Phyllis and her sisters mimicked the singing style of the Andrews Sisters, as well as the Mills Brothers and the Dinning Sisters. From very young ages when they would perform short shows for family and friends in their parents' living room.