According to the IRS publication 501 - www.irs.gov/pub/irs-pdf/p501.pdf There are five tests that must be met for a child to be your qualifying child. The five tests are:
4. Support, and
5. Joint return.
To meet Age Test t, a child must be:
Thus - regardless of her age - because of disability - your daughter is your qualified child - assuming other tests met.
There is no limit on earnings for the Qualifying Child determination.
There are four tests that must be met for a person to be your qualifying relative. The four tests are:
1. Not a qualifying child test,
2. Member of household or relationship test,
3. Gross income test - here is a limit on earnings
4. Support test.
However because your daugher is your Qualifying Child - she may not be your qualifying relative - see teh first test.
So - regardless of what should be included into the income to be a qualifying relative - that doesn't affect your eligibilty to clain your daugher as a dependent if she meets all five tests to be your qualifying child.
Please let me know if you need any help or clarification.