There has always been a pretty enflamed division between East Slavs like in the Kingdom of Ruthenia / Galicia / Volhynia/ Poland (Catholic) and West Slavs (Orthodox). Yes, the Rurikids were on both sides of that divide (east until 1323 and West through Ivan the Terrible) and yes, the same genetics. Same cultural and national backgrounds from a 21st century perspective? I don’t think so - they hate each other too much. Kind of like saying the English and Irish are the same culture and nationality. That’s how I see it.
You and I are too far apart on this. I’ve read a lot of the history. Have you read Bloodlands?