26 C.F.R. § 1.662(a)-4
Any amount which, pursuant to the terms of a will or trust instrument, is used in full or partial discharge or satisfaction of a legal obligation of any person is included in the gross income of such person under section 662(a) (1) or (2), whichever is applicable, as though directly distributed to him as a beneficiary, except in cases to which section 71 (relating to alimony payments) or section 682 (relating to income of a trust in case of divorce, etc.) applies. The term legal obligation includes a legal obligation to support another person if, and only if, the obligation is not affected by the adequacy of the dependent's own resources. For example, a parent has a "legal obligation" within the meaning of the preceding sentence to support his minor child if under local law property or income from property owned by the child cannot be used for his support so long as his parent is able to support him. On the other hand, if under local law a mother may use the resources of a child for the child's support in lieu of supporting him herself, no obligation of support exists within the meaning of this paragraph, whether or not income is actually used for support. Similarly, since under local law a child ordinarily is obligated to support his parent only if the parent's earnings and resources are insufficient for the purpose, no obligation exists whether or not the parent's earnings and resources are sufficient. In any event the amount of trust income which is included in the gross income of a person obligated to support a dependent is limited by the extent of his legal obligation under local law. In the case of a parent's obligation to support his child, to the extent that the parent's legal obligation of support, including education, is determined under local law by the family's station in life and by the means of the parent, it is to be determined without consideration of the trust income in question.
26 C.F.R. §1.662(a)-4