She's Not Our Daughter!

A few years ago, my ex-boyfriend, whom I broke up with after our one-night stand, came to visit me as a duke. He said he regretted abandoning me. What are you talking about? What’s there to regret when we’ve split up with ease? Also, I think he even misunderstood my sister’s daughter for ours.
“I had no idea. I didn’t know you were raising our child alone.”
What is this situation? Just then, my niece, who hid herself behind my back, stuck her tongue out at him.
“I hate you, you’re a bad guy who abandoned us. Go away!”
He staggered in a shocked manner No, he didn’t abandon you. And I’m not your mother!