-if you S Rank a Hoshido sibling, they either find out through a letter left by Mikoto or Ryoma just says it to Corrin that they are not actually blood related. Revelations reveals from where Corrin and Mikoto come from and who was her first husband but this is never discussed in BR
-Valla appears in one chapter in Conquest(which its only real purpose is to re-recruit Gunter), it's never mentioned again and Azura doesn't even bother to explain what it is to Corrin unlike how she does in R's Chapter 7
-Azura is revealed to not be blood related to the Nohrian siblings in C but where she and her mother come from is also never explained.
-the actual reason Corrin can transform into a dragon is not even discussed, it's just accepted as if it was a normal thing like shooting lasers from your sword or killing people with trees
-who and what possess both Takumi and Garon in C
-Lillith's origins
-The Hooded boss in Chapter 5 who even has "???" for his name(spoiler: it's Sumeragi, why do you think he's a Swordmaster?)
And even better, plot points that make for a very unsatisfying ending without them and are mutually exclusive to BR and C:
-The fact that Azura's song slowly kills her is never mentioned even when the circumstances that Corrin finds out about it in BR are the same as in C(after the Opera House chapter, Corrin follows Azura to a lake, in BR they see her curse take effect, in C this leads to them... following Azura through a portal to Valla and the curse's effects are never mentioned
The result: Azura disappears at the end of Conquest and no one knows ever finds out what happened to her
-King Garon is actually dead and his body is being used as a puppet: this is revealed by Azura to Corrin at the end of Chapter 15 in C, her proof is a Crystal Ball that reveals the truth to a, quote-on-quote, "someone with dragon's blood or a very powerful mage"
They only ever bother to reveal this to the others 11 chapters later when Leo could have used said crystal and tell the others about it(as the others siblings needed more to be convinced than Corrin was)
Azura never bothers to tell this to anyone in BR
The result: Garon will be remembered in history as a tyrant when the real Garon was actually a caring father and husband to all of his children and concubines
Also, as a side note, the real Garon is never revived in Revelations unlike all the other royals(Mikoto, Sumeragi and Azura's mother)