I've done this with two females. A seven year old and a 7 week old. Our older girl really didn't think the puppy was a necessary addition, and didn't like her at all initially, but since it was a puppy she tolerated her. Generally she would growl and walk away, even from her feed dish, allowing the pup to have it. After about three days of hoping the pup would disappear, she gave up and started playing with her. After a few weeks, she began treating the pup as if it was her own. The first time we introduced the pup into our bed, however, the older girl was seriously put out and didn't want that youngster to even touch her! Later on, however, they would sleep curled up together and were inseparable after that. The only time they squabbled is if I had had the youngster out to a show and was away for a couple of days. Then they would stand on their hind legs, lock arms, and snarl into each other's faces. But that was it. Back to normal.
I always made sure to give preference to the older girl. After all, she was there first! Given that yours are opposite sex, it probably won't take too long before everyone adjusts.