I think that, for the timebeing, I would shelf
Isis. Especially for a villain, it seems redundant and uninspired even if you named her before the organization was ever heard of. I hope someday soon we can hear and see this name without thinking of such an evil group.
Personally, the first thing that comes to mind when I hear
Isis is the dog from Downton
Abbey, but it's directly undermined by the second-- the terrorist group.
“Fairy tales are more than true; not because they tell us that dragons exist, but because they tell us that dragons can be beaten.”
― Neil Gaiman