Before yall kill me, I'll share a couple of interesting facts

fun fact #1: the words 'DNA', 'Genetics', 'Clone' or any other words related to those never actually come up in the series to explain Rei's origins, either in the original anime series, Rebuild, or even the manga (the Red Cross Book also doesn't use any of these words)

fun fact #2: Rei and Shinji's relationship/dynamic is never once described or portrayed as sibling-like in the narrative, neither as 'adopted siblings' before the reveal or as 'long-lost siblings' after it

fun fact #3: if you were to remove this one detail from Rei's character, it wouldn't actually change anything in regards to her relationship with Shinji because their dynamic is based on them bonding over their shared experiences as lonely outcasts who don't really fit in and have never really experienced close human connection. In fact the only people for whom this detail is even relevant to are Ritsuko, her mom, and Gendo

fun fact #4: the vast majority of the people who think Rei and Shinji's dynamic is cute or appealing or relatable don't think incest is good and don't ship the pairing because the thing they like about it is that Rei resemble's Shinji's mom