I think the ideal family for Reiji would be: Karlheinz as father, Cordelia as mother, and the triplets as younger brothers.
Just think about it.
Only Reiji prioritizes Karlheinz's orders. Monthly dinner? He'll be there. He even cooks the food himself.
In Reiji's route, we learn that he wanted/dreamed of being Cordelia's son. Looking at her warm relationship with the children (the triplets), and also the fact that she never behaved inappropriately toward Karlheinz's children from other wives.
And seeing the coldness of his own mother (Beatrice), he longed for the warmth that the triplets had from Cordelia. A typical childhood wish.
With the triplets, it's also very easy to see. Reiji's relationship with Shu is filled with hatred — he even kills him or wounds Yui.
Toward the triplets themselves, Reiji has no negative emotions; on the contrary, he simply disciplines them like an older brother would younger ones. I think he even enjoys Laito and Ayato's mischief, and Kanato is also important to him.
And precisely from Reiji's own attitude toward Cordelia's family, his desire to be a member (not indirectly, but directly) can be seen very clearly.










