He is not her brother by blood. He is also someone who was brought to impersonate the second son.
W-w-WHAT HAVE I BEEN SKIPPING TO MUCH OR HIT MY HEAD WHEN DID THIS OCCUR
No, I think it was merely implied that he is the product of the duchess having an affair, not an impersonator. It hasn't been stated outright, but that's how I understood it.
Even if what you said might be true, FL is still not related to the brother by blood. She was picked up from the streets.
FL is the Duke's illegitimate daughter, so if the Brothers' Father is the Duke, they are still half-sibling and blood related.
I don’t want to raise any alarms but her brOTHER IS OBSESSED RED FLAGS GUYS