As a nurse, I often relied on my mother-in-law, Denise, to watch our son, Leo. She could be overbearing, but she lived nearby and rarely said no. Lately, though, Leo started clinging to me and hiding when she arrived. One night before a shift, he burst into tears: “I don’t want Grandma to stay. She acts strange.”

After a sleepless night, he admitted she’d been chasing him with cotton swabs, saying she needed his spit “for a tube.” My stomach dropped. I woke Denise and confronted her. She confessed she’d been trying to get Leo’s DNA because his blond hair made her doubt he was Andrew’s son. I asked her to leave.

Denise called Andrew on her way home, planting doubt. He asked for a test “to settle it.” I agreed—on one condition: he’d test, too, to confirm his father. If Denise wanted certainty, she could face it as well.

Results: Leo is Andrew’s son. But Andrew’s test showed his biological father isn’t the man who raised him. Denise finally admitted to an affair years ago and said she’d always suspected it but never tested while her husband was alive.

Andrew was devastated. We decided to step back from Denise and focus on our family. Her guilt had been eating her for decades—and she projected it onto me and our child.

What would you have done?