You need to buy some bitter apple spray and spray the tails of the kittens or cats that she is sucking on. Give her an alternative object to suckle, a small stuffed animal, or make up a rice heater and let her suck on that.
But is it bad for cats to do that? I mean, if the others don't care, should I consider it a problem? Because my two kitties are doing it, to each other and themselves...I always figured it was just like, brotherly love, comfort or something...