I am the supervisor of 4 secretaries. A year ago I hired a secretary who had come for a supervisory position.?

I am the supervisor of 4 secretaries. A year ago I hired a secretary who had come for a supervisory position. In this last year she has done nothing but complain about everyone one here. She has buddied up with another secretary who has a wonderful history of quality work and working as a group. Since she has gotten in close with "Maria". Her performance has dropped, her quality has dropped, she spends alot of time whispering and giggling with Maria. When I call a meeting to inform them of new information, they complain about how the rules always change and make demands of what they need to do the job. Comments are made like.I was never told that.etc. Maria does not want to hear instruction from me and cuts me off short then goes and tries to turn the other sec against me. It makes for a very bad uneasy environment and I hate that this new sec has poisoned our group. Everyone was happy and got along great until she came. I want to talk to the other secretary who is also now acting like I am a monster and let her know what is going on. But I don't want to risk her telling Maria and Maria trying to take it to an ugly place. Should I try to talk to "Jackie" or let her kleep going down? Don't want to do anythign illegal and loose my job.