No, your first daughter does not need to file Form 8615. Your second daughter would be required to include it on her return, but there might be another way that may be easier.
Form 8615 will be attached to each child's return to correctly calculate the taxes due on their returns. The form uses your tax rate to tax their unearned income. This is to close a loophole that allowed parents to pay their child's tax rates on things like dividends and capital gains.
Since your first daughter is no longer filing as your dependent, she does not have to pay taxes at your tax rates. As you said, to qualify as a full-time student, she has to be full-time for any part of five months of the tax year. She falls just short if she only went from September through December, assuming she was not attending a different school in the earlier part of 2021.
An alternate method for your second daughter may be to include her income on your return. Form 8814 is used if you include all the income from your second daughter on your own return. However, this can only be used if your daughter's income is solely from interest and dividends and their total income is under $11,000. If your second daughter meets those eligibility requirements, she would not have to file her own return. For more information on reporting your children's income, see this IRS tax topic.
