Unfortunately, not claiming them as a dependent doesn't work. Even though their overall income may be enough to say they provide half of their own support, if they don't have enough earned income to provide at least half of their own support, they are still taxed at the Estate and Trusts rate. This is explained in the following reference: Form 8615 Instructions (a pdf link). You will see this information on page 1:
Who Must File
Form 8615 must be filed for anyone who meets all of the following conditions.
1. You had more than $2,100 of unearned income.
2. You are required to file a tax return.
3. You were either:
a. Under age 18 at the end of 2018,
b. Age 18 at the end of 2018 and didn’t have earned income that was more than half of your support, or
c. A full-time student at least age 19 and under age 24 at the end of 2018 and didn’t have earned income that was more than half of your support.
(Earned income is defined later. Support is defined below.)
4. At least one of your parents was alive at the end of 2018.
5. You don’t file a joint return for 2018.
These rules apply if you’re legally adopted and a stepchild. These rules also apply whether or not you’re a dependent. These rules don't apply if neither of your parents were living at the end of the year. (Bolding for emphasis)
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