If a parent COULD claim her as a dependent, then she should have been marking that she could be claimed as a dependent on someone else's return. If a parent COULD claim her, she's not eligible for the check.
It's impossible to tell from this who has been filing incorrect returns. Both can file returns, on paper in the mail, claiming the child, and let the IRS sort it out. Then the stimulus payment can be sorted out in 2010 on their tax returns. For purposes of the stimulus, it's too late now for 2019.
