Your father had begotten Sikhandin for the slaughter of Bhishma. As regards Arjuna he only protected your brother when he caused the death of the high-souled Bhishma.
स चापि सृष्टः पित्रा ते भीष्मस्यान्तकरः किल॥ शिखण्डी रक्षितस्तेन स च मृत्युर्महात्मनः।