This is an unusual situation. How old is your granddaughter? Typically, this happens when parents have an adult child (in college under the age of 26). If the parents are not claiming the dependent, then the 1095-A is apportioned by 3 and the dependent will claim their portion on their return.
I believe that this would be true for the 1095-A in your case. You would need to enter her portion of the 1095-A form into your return. If you are claiming her, you should ask her parents to provide you with a copy of the 1095-A. They should report 2/3rds of the amount and you report 1/3rd of the amount. The problem you might find is if your income is above the level to which her parents qualified for marketplace insurance and any stimulus, you may have to repay the portion of the subsidy (if any) that is attributable to her.
There is another way to divide up the amount which is by percentage. The 1095-A will ask questions regarding the proper percentage. How is shared policy allocation calculated? In the event your client cannot agree upon a reasonable allocation percentage, the allocation percentage will typically be equal to the number of individuals on the shared policy that your client can claim on their tax return, divided by the total number of individuals on the policy.
It may be possible that you may not be able to e-file your return.
On parents 1095-A but not on their return
