The aunt, if she is really disabled.
See publication 501. To be a qualifying child dependent, the child must be permanently and totally disabled. For this rule, disabled means unable to perform gainful work, that is permanent or will last at least one year. Being able to work a minimum wage job without special accommodations is being able to do gainful work. This means that some people with medical disabilities who can still work (like blindness or paraplegia, for example) are not disabled for tax purposes. It also means that if someone can't work their chosen job, but could work a different job, they are not disabled for tax purposes even though they might qualify for disability insurance payments from their original job.
If your child is permanently and totally disabled, then she is a qualifying child dependent of the relative (parent, grandparent, sibling, or parent's sibling) where she lived more than half the year. The dollar value of support is not considered. Because she lived with the aunt more than half the year, only the aunt can claim her as a qualifying child dependent.
If she is not permanently and totally disabled, then qualifying child dependent is out and you have to turn to the rules for qualifying relative dependent. Here, the first test is that your child can't have more than $5500 of taxable income in 2025. If she has taxable income (from wages before becoming disabled, pension, investments, or taxable disability payments) then she can't be a qualifying relative of anyone. If she does not have more than $5500 of taxable income, then she can be a qualifying relative dependent of anyone who paid more than half her living expenses. The person who paid more than half her living expenses does not have to be a relative, and the child does not have to live with the other person.
That means that you, the father or the aunt could possibly claim her as a dependent. But, you have to add up all her support costs, including food, rent or mortgage, medical care, travel, entertainment, clothes, and so on. If she is the only other person living with her aunt, then for those 9 months, the aunt gets credit for half her mortgage, utilities, insurance, household maintenance, food, and other expenses, plus whatever she paid for clothing, medical care, travel and other personal expenses for your daughter. You get credit for 3 months of shared living expenses (half your rent, etc. if there were just two of you; 1/3 the expenses if there were three of you, etc.). Your ex gets credit for medical expenses he paid. Whoever paid the most overall is the person allowed to claim her as a dependent. If no one paid more than half, then no one claims her as a dependent unless everyone who contributed part of her support signs a statement that agrees as to which person can claim her.
But this math only comes into play if she is not a qualifying child dependent by reason of disability. If she is disabled, the aunt gets to claim her even if someone else paid more than half her total expenses.
