He's not her biological brother. Anissa meets her biological brother much later on in the story. (It says this on the novel blurb, so it's not even a spoiler.)
I want to say that Diedrich's biological sister survived, too, and it was a baby swap deal, but I could be thinking of a different story. (I didn't read the spoilers very closely.) But Anissa is definitely another woman's child who was abducted/liberated from her biological parents. It literally says this in chapter one.
Correction to my last post: The girl I was thinking of wasn't Diedrich's sister (she's almost certainly dead), but the novel heroine, Charlotte. She's obviously alive. I got them mixed up because she has a patron/family-like relationship with Anissa's biological family. (And again, I didn't read the spoilers very closely, hence the confusion.)

The tag say romance..
Does that mean diestrich is ml?...