First of all you have to ask the question Will you get married without your Dad? if the answer is
yes then you need to tell you dad that the ball is in his court either he comes alone or not at all.
If the answer is no and your dad is unwilling to go without her then you will have to allow her to come but put plenty of conditions in place, like she must stay away from your future mother in law etc.
So at the end of the day it all depends on how desperatly you want your dad there (which i know
every girl wants their daddy at their wedding)
hope that helps :)