If they file a joint return you can't claim either person as a dependent.
If the husband were to file as "married filing separately," you might qualify to claim your daughter as a dependent. This would be worth a $500 tax credit only**, and that might be less than the extra tax the husband would pay on his income. You would have to prepare some test returns with different situations.
**If you are unmarried, you might qualify to file as head of household, which could save some more tax. If you pay for medical expenses for your daughter, you may be able to deduct them on your tax return if you claim her as a dependent.
