New kitten in the house?

Alright...I started out with two cats (Pepper and Ginger) they are both around 3 years old now. Just a couple weeks ago, my mother found a poor baby kitten in the road and we took her in. (we named her Abby) At first she stayed locked up in one room while she was recovering from numerous health problems. About a week ago we let her out...and now she roams free. The problem is Pepper, who is my baby girl, always has slept at the foot of my bed, but now Abby keeps coming in my room, chasing her, swatting at her, and making tons of noise, then she takes Pepper's spot, and I feel SO bad. I can't lock Abby out of my room though because Ginger likes to attack her and stalk her and every time i put Abby off the bed she jumps back on...relentlessly, until I just have to deal with it. What can I do to tell Abby that she's not the boss?