This is interesting. They were the new age Clark Sisters. That's a pretty good comparison. As far as Mary, they were calling MJB "Hip-Hop Soul", while Toni was straight rhythm and blues. Good analysis. That was the new jack swing era. GenX was just starting to create their own derivative of RnB. RnB got more "Edgy" in 1991 with Jodeci's debut.
Nowadays, our generation(those of us born between 84-00) is starting to create the next derivative of RnB, with this "Trap-Soul/Trap-N-B" era. Similar to the creation of New Jack Swing 25-30yrs earlier. But lyrically WAY more Edgy. Nowadays we got singers literally crooning about trapin, holding pistols.