Help! My cats are not warming up to new cat!?

I have two cats. One is Lola who is 3 years old and the other is her son MoMo who is one year old. I have had them since MoMo was 2 months old. Recently, I found a sweet cat in the streets. It started snowing here so I took her in and decided I was going to take care of her. I knew my cats would object to her but I didn't realize it was so hard. At first I kept the new cat, Sweety, in my room with food and litter. Every once awhile I took her out. Lola would attack her on the spot. Mo, well he went up to Sweety to say hi but she hissed at him. Everytime Lola and Sweety fight, MoMo gets real scared. He runs away or hides. He walks real slow all the time and I haven't seen him eat. Also, Lola attacks Mo and hisses at him. Its been six days and I am not sure what to do. I love the new cat a lot but my Lola is stressed out completely. I want to keep Sweety. She is very sweet to me and way adorable but I am just not sure what to do. Any suggestions?