Steven Levenson’s excellent play “If I Forget,” currently running at Victory Gardens Theatre in Lincoln Park through July 7, is about three self-centered siblings who have reunited to attend the 75th birthday of their ailing, widowed father.It has been compared by some to Tracy Letts’ “August: Osage County” as yet another example of the dysfunctional family in America today.I do not agree. It is my understanding that a “dysfunctional” family is afflicted, among other things …