First off I should mention that you are NOT SUPPOSED TO KILL THEM. USE SANTUARY. Santuary can be found in Sandy Grave after getting Toad Morph.If you really want to kill them and get the bad ending go ahead. I suggest targetting Loretta as she has lower defense. Stella and Loretta mainly use combination attacks. If Loretta says "Sister" then Stella says "Ready sister!" get behind Stella as she is about to hit you with a massive Ice Sword that does more damage than Death's ultimate attack. Sometimes
Loretta causes a big ice crystal to move around the room. You can avoid it by ducking but I suggest running and slidding because Stella will eventually shatter it into a bunch of pieces that fly everywhere hurting you if you get hit. She also causes a portal to appear on the ceiling and follow you around dropping icicles. You must attack Lorreta several times to stop this attack. Finally Loretta may use Freeze Bites where she opens a portal and several icy lion heads fly at you. The best thing you can do
here is just slide or get behind her. If you want to actually pass this fight and move on with the game place Charlotte somewhere near the sisters
and have her cast Santuary. It takes forever to cast so keep the sisters' attention toward Johnathan by using Taunt. Alternativly you could just
wait until they start their ice-sword attack. Stella does this three times and if you can keep her turned away from Charlotte as she does the attacks
Charlotte should be 2 seconds from casting Santuary just as the third ice sword hits. Good luck.