I have a daughter, she's 6. I am not biologically related to her. There is no blood relation between us.

I was best friends with her mother for most of my life. We were never romantically involved — just close friends. She had her daughter with her husband, and when their daughter was a newborn, both her parents were tragically killed in a workplace shooting. They had no siblings and came from rough family backgrounds, so there was no one else for their baby to go to. Since I was close to them and knew the situation, I stepped in and was able to adopt her.

I never planned to raise a child, but I couldn’t let her end up in foster care or with strangers. I loved her parents and couldn’t just walk away from their daughter. So I became her father.

Everyone close to me knows she’s adopted. I’ve always been honest about it. My daughter, who I’ll call Lily, knows her parents are gone and that she was adopted, though I’ve saved the full story for when she’s older. Lily doesn’t resemble me at all — she looks exactly like her mother and biological father. A lot of people just assume she’s mine and took after her mom. I don’t usually correct them unless it feels necessary.

Recently, my brother’s fiancée (my SIL-to-be) saw an old photo of me with Lily’s parents and asked who they were. I explained they were Lily’s parents. She didn’t say much, but she must’ve assumed Lily was actually my biological daughter and that I was hiding something.

Apparently, she used my brother’s DNA to secretly do a test on Lily. When the results showed they weren’t related, she realized I wasn’t Lily’s biological father. She came up to me, in front of my daughter, waving the results and saying I was raising someone else’s kid. I just stared at her in disbelief and then started laughing. I couldn’t help it — it was so absurd and inappropriate.

I told her I know Lily isn’t my biological child — that’s literally how adoption works. She turned bright red and stormed off. Later, my brother said she was embarrassed and called me a jerk for laughing. But what did she expect? She secretly tested my child’s DNA like she was exposing some scandal and then tried to shame me over it. For what? Loving a kid who needed a parent?

I honestly don’t feel like I was in the wrong here. If anything, I think she was out of line and incredibly disrespectful. So, Reddit, AITA?