I don't see a conflict of interests at all with your continuing to advise the group, any more than if a non-Greek or a fraternity man advises a sorority (which is quite common). The only way it would be a conflict is if they wanted to go with a national other than yours or stay local and you steered them away from it, which you definitely aren't doing. You are being one fab advisor, because your main concern is the girls' sisterhood and happiness!
I have no idea what it is like to be at a school w/ over 60 in a chapter, let alone twice that
but from what I've gathered on here, the bigger you get the more you want to be sure you stay that way. One of the keys might be to show **consistent** overage in recruitment over many years, to let the other groups know this isn't a fluke.
Definitely tell them to keep on truckin' for now, and enjoy sorority life locally without waiting on Panhel. If everyone's at total, and the other groups keep denying them the chance, the other groups will (to be blunt) just look like they are being bitchy. They need to "share the wealth" as it were! GOOD LUCK!!