"In addition, we got a stimulus check for my mom in April. She passed in 2018 and we had to return the funds."

Because the payouts are Advances, projected using 2018 or 2019 tax filings. Then, the 2020 Actuals is the reconciliation for what you are entitled to against what you got already.

So, of course, Mom doesn't qualify, but MIL qualifies.

"We don't need the money."

Give it to your Food Bank.