Yes, your mother-in-law is likely required to file a tax return since she received a 1099-S and has wages in addition to Social Security. She reports the full home sale on her own return, and may qualify for the $250,000 home-sale exclusion if it was her primary residence.
On your joint return, your husband generally reports the 1099-S and then removes it as nominee income if he had no beneficial ownership.
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