How do I keep 1 female Zebra finch from attacking my other female zebra finch??

We have had them for 7 months without any problems until now. They have Kaytee Forti-diet Egg-Cite (finch Food). They also get Kaytee Hi-Cal Grit for finches. If we cover the cage the aggressive finch stops being aggressive. She is pulling out her sister's feathers.