@Steph-B- - please review the instructions for the solar credit (link below) on page 1. It appears the son-in-law must live in the home to be eligible for the credit.
Who Can Take the Credits
You may be able to take the credits if you made energy saving improvements to your home located in the United States in
2021.
Home. A home is where you lived in 2021 and can include a house, houseboat, mobile home, cooperative apartment,
condominium, and a manufactured home that conforms to Federal Manufactured Home Construction and Safety
Standards
I caveot this answer in that Biden approved the Inflation Reduction Act today which has impacts on the solar credit. I do not know if this new act changes who can claim the solar credit.
https://www.irs.gov/pub/irs-pdf/i5695.pdf
