This study analyzes the family holding company from the perspective of the tension between private autonomy and fiscal justice, focusing on the constitutional limits of estate and tax planning. The research problem consists of determining to what extent the reorganization of assets through a holding structure constitutes a legitimate exercise of private autonomy and under what circumstances it may represent an abuse of legal form or a violation of the principle of ability to pay, particularly in relation to the Tax on Inheritance and Donations (ITCMD). The research adopts a qualitative, legal-dogmatic approach, based on the analysis of the Federal Constitution, the National Tax Code, legal doctrine, and the case law of the Federal Supreme Court and the Superior Court of Justice. The results demonstrate that the family holding company constitutes a legally legitimate instrument for estate planning when structured with a genuine business purpose, effective economic substance, real patrimonial separation, and tax transparency. The distinction between lawful tax avoidance and abuse of legal form does not derive from the formal structure itself, but from the correspondence between legal form and economic reality. It is concluded that the disregard of the holding structure must be exceptional and conditioned upon concrete evidence of simulation, dissimulation, or artificial legal arrangements, and that mere tax savings are not sufficient to characterize illegality.