From the looks of it, she probably is. When we initially saw the person labeled as her father when her and ML were attacked, he had silver hair and purple eyes. The exact same thing the demon king did. Not to mention he used magic. There’s also ML not being shocked at how natural things regarding magic a demonic beasts are for her but then tells everyone he can’t say anything yet. They also said that the last time the demon king was seen that he looked like he wanted to say something but didn’t. He likely wanted to bring up that he had a child with a human and didn’t want her to be exposed to demons and dealing with such a cruel role. From the looks of it, he also cast a spell to prevent MC from remembering him.
Could it be that she is the daughter of the demon king but doesn't know it?