I've seen Katelyn is Neil's secret sister fics. What about Jeremy as Neil's secret brother? Jeremy's mom had a fling with Nathan, realised he was in to shady stuff, left him, had Jeremy (Nathan never knew she was pregnant), and married the congressman step dad. Little Nathaniel was born two years later to Nathan and Mary. Jeremy was told his biological father was not a nice man and to never ask about it or it could cause problems for his step dads political career.
It all comes out, and Neil is like....damn nurture wins, I guess (except maybe it doesn't cause Jeremy is sus as hell just like Neil)
Andrew, Cat and Laila finds this VERY funny
Jeremy, Jean and Kevin are horrified.
Also, Jeremy and Neil lowkey do not super like each other, so....
Anyone is free to take this idea and do what you will with it.