Responses
I think it's the son because his hair is a different color. Isabella and Arabella had the same color hair.
The sons hair is i think the same shade as the cardinal, also arabella and i word(isabela) have the same hair color bc they share the same mother
I bet the affair child was Isabella (and might be the eldest son too lmao) especially the freaking mother mentioned she was forced by someone to live her whole life with an insatiable man. The freaking mother hates Arabella because she's the only child that connects with the Cardinal. Somewhat Ariadne & Arabella share the same eyes.