It won’t come up often, but will certainly help in certain situations. The general rule is that eligibility for SSI disability payments, and SSI-related Medicaid, for minor children depends on the income and assets of the parents, which are "deemed" to be available to the child. "Parents" include "step-parents." But only the income and assets of a parent or step-parent who resides with the child are deemed against that child’s eligibility for disability benefits and Medicaid.
An unusual situation arises when the natural parent of a child terminates the relationship with the step-parent, moves out of the family home, but leaves the child living with the step-parent. For years, SSA’s position was that even where the parent-step-parent relationship ended, the child lost eligibility for SSI disability benefits through deeming of the (former) step-parent.
The courts did not agree. In Florez v. Callahan, the Second Circuit reversed SSA’s position:
"The plaintiff stepfather took on the care and support of his emotionally disabled stepson after his wife, the child’s mother, abandoned her family. When the stepfather applied for SSI disability benefits on behalf of his stepson, the Social Security Administration….
"Plaintiff, in assuming the sole responsibility of caring for his wife’s child after she left home, shows himself to be a person who plainly believes that in passing through life, any kindness he can show to another must be shown now, and not put off until another day. One would suppose that a social services agency would encourage such a generous attitude. But, the Social Security Administration adopted quite the opposite position and penalized the stepfather by ruling that his income, prior to the child’s entering the psychiatric center, was attributable to the child and thereby reduced the amount of monthly SSI benefits. The stepfather appeals this first ruling, and also appeals a second ruling that interpreted the regulations to authorize a reduced flat-rate payment of SSI benefits once his stepson was admitted to the medical care facility. "
The court reversed SSA’s rule in 1998, at least for residents of the Second Circuit. On May 15, 2008, the Social Security Administration finally nationalized the rule adopted by the court and issued a new regulation, modifying 20 CFR 416.1160.