Couple of additional points.
1. For the gross income test, some expenses are subtracted to determine if the gross income is more or less than $4250. It depends on the type of work and the kind of expense. See publication 501 page 18.
2. If you claim your parents as dependents, you would get a $500 tax credit per person, plus you can deduct medical expenses that you paid. You don't get a stimulus payment for them under the current law. There has been discussion of a second stimulus with different rules but nothing has been passed by Congress yet.
3. If you can't claim your parents as dependents due to the gross income test or the joint tax return test, but you still provide more than half the total support, you can still deduct medical expenses that you paid for them even though they are not tax dependents. See publication 502 page 3. https://www.irs.gov/pub/irs-pdf/p502.pdf
