Sakura and Takumi are Mikoto's and Sumeragi's kids. Hinoka and Ryoma come from previous marriages.
Sakura and Takumi, along with this, are the two closest siblings. Takumi felt distant from Ryoma, and Takumi feels like Hinoka doesn't give a damn about her ever since she tried to run off to Nohr to find Corrin, leaving the younger siblings behind.
He has a grudge against her for that.
Much like how Sumeragi handed down Raijin, Mikoto was the one who handed Fujin down to Takumi. She's the one who taught him how to use a bow. She made sure he knew how to use a normal bow in the case that Fujin was lost. She also taught Sakura and how to use rods to their full potential. They spent the most time with Mikoto and ended up being hurt the most when she died.
Takumi's bad dreams are a well guarded secret among the army. Four people know this - Mikoto (alas, she is... well, you know), Sakura, Oboro, and Camilla. Sakura and Camilla from experience, Oboro from an accident and mutual bonding.