First of all just want you to know I am very interested in wicca and up till now have only been reading about it. I find myself now trying to write a script for a film about it. Its the good against bad you know the norm but please don't worry it will be great. The only thing is I need the four sisters (who are the good ones) to invoke a good gaurdian so they have the strength against the black magick. Please could someone post me or email me how and what to say to invoke the good force. The only thing is the weakest of the sister does turn to help the evil so how could this happen if she has invoked the good she only helps him cause she is scared. please help.......

in short, your Idea is completely ridiculous, I dont mean that to be rude its just that the fundamental(main) points of your plot contradict each other.
first, there is no black/white magick. magick is a tool in the same way as electricity. it is neither good nor bad, the intent behind it, that is to say, the way it is used is good or bad.
Secondly if the 'weak' sister had invoked a guardian, and done it right then there is no reason why any negative magick should affect her as she'd have invoked the guardian for the simple means of protection.
lastly, if you are interested in the way magick actually works then email me and I can give the little that I know also, Kate West is an incredibly good author on the subject.

blessed be.