Hi Michelle,swap toys,blankets and then short supervised meet and greet,Amber just needs to confirm that having a new sister doesn't mean any major changes to her existing life and that actually it could be quite fun,and once she has told Millie in no uncertain terms that she (Amber) is in fact as always topcat in the house and assuming Millie accepts this then thats half the problem solved,oh maybe also swap a little of Millies used litter into Ambers and vica versaGood luck and don't worry to much it just takes a little time sometimes