I think all four of the Hoshidan siblings are Sumeragi's and Ikona's children, so I think that Takumi was probably 1 or 2 years old and Sakura was either a couple of months old or 1 year old by the time Corrinmui was kidnapped by Garon, which would explain why Takumi and Sakura barely remembers Corrinmui when he/she reunite with the Hoshidan siblings. As for the Nohrian siblings, they're all the children of Garon, but Marxander mother is former queen Katerina, with Camilla's, Leo's, and Elise's mothers being individual concubines. So essentially they are all half-siblings.