This is copied directly from the 2020 PA Tax Instruction Booklet:
(J) Married, Filing Jointly
You and your spouse, even if living apart or one of you is deceased, can
file a joint return for convenience. The executor(s) of the estate(s) of the
taxpayer and spouse may also elect to file jointly. To file jointly, you must
meet ALL of the following conditions:
1. Your taxable years end on the same date (not including returns with
a deceased taxpayer and/or spouse); and
2. You and your spouse elect to have the same residency period
(earliest starting date if you moved into Pennsylvania and latest
ending date if you moved out of Pennsylvania) if you are part-year
residents; and
3. Neither of you is individually claiming one or more of the credits on
PA Schedule OC, (see Page 22); and
4. Your deceased spouse’s estate has not elected to file a separate
return on his or her behalf; and
5. Neither of you is individually liable for the payment of child or spousal
support, or another liability to the PA Department of Human Services;
and
6. Neither of you is individually liable for the payment of a court-ordered
obligation arising from a criminal prosecution or proceeding
Page 6,
https://www.revenue.pa.gov/FormsandPublications/FormsforIndividuals/PIT/Documents/2020/2020_pa-40in....
If your grandparents qualify under these criteria, I'd suggest going back to the My Info section of TurboTax and designating your grandmother as the primary taxpayer, by entering her identifying information first.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
