Section 7. In actions for the construction of wills, or for instructions relative to wills the court may order notice of the action and of the time and place for hearing to be served on such number of the parties in interest representing all possible interests as the court shall direct, and to be published for three weeks successively in such newspaper as it directs. If it appears that any possible interest is not represented, further service may be ordered until all possible interests are represented before the court or until a guardian ad litem has been appointed. If all possible interests are represented by persons before the court, it shall be unnecessary to make other persons having similar interests parties defendant.