There was a previous thread relating to this saga in which Joni is quoted as stating that while the papal performance was arranged by the Joni, Debbie, Kim configeration, Kathy was approached to participate. According to Joni, Kathy responded days before the performance with a list of demands such as having her own dressing room, using her own band, renaming the performance as Sister Sledge featuring Kathy Sledge etc etc. Apparently the sisters thought this too much and did not pursue the matter with Kathy.
Regardless of who is right or wrong,I agree with other comments that this is a sorry saga which tends to overshadow the remarkable achievements of these 4 talented women.