Loving all the Bertrand questions! Unfortunately, if Bertrand wasn't killed when Isabelle was an infant, the strain between him and Alec would only lead to more distance and it would only be a matter of time before it came to a head—probably with Bertrand trying something with Isabelle and Alec either killing him then or asserting dominance in a way that would make it very clear he's not the pushover little brother anymore. Bertrand doesn't have it in him to submit and would struggle to accept equality. Their relationship would never be as close as it once was, and the power struggle would either force them apart or turn very destructive.

It occurs to me that that would be a great setup for Alec moving to the US....

Oh, God, that's a disturbing thought to me. But then, it all is! I don't think Bertrand has the temperament for parenthood like he would like to think. Mourning Alec, he'd look for him in Isabelle, and failing to find him there would leave him frustrated with and distant from her. There would be a hot and cold of him pushing her away and making attempts to bond and recover something of the bond he and Alec used to have, and eventual sexual abuse in her teens, but not as comprehensive grooming as Alec inflicted on her. Importantly, Bertrand also wouldn't discourage Isabelle's independence the way Alec did, and sometime in her late teens or early twenties she'd be unleashed as a "deeply dysfunctional but still able to get by" adult.

Of course, they were babies together before anything else. The bond they formed in their, well, formative years, was the foundation for everything that came after. They were always, always there for each other, no matter what else. Bertrand took the role of leader very very early, from about four years old onward to adulthood, just because he was apparently (lmao not really) better at emotional regulation than Alec.

When they were six or seven, Bertrand got tired of Alec always being guilt-ridden and angry and in trouble with the adults around them, so he started teaching Alec how to get away with things and trying to convince him that guilt was a useless emotion best discarded. Eventually this worked. In secondary school the two of them were fucking terrors, getting away with everything short of murder. Those were sort of their golden years, in their late teens. They were best friends, lovers, and partners in crime.

Then, when Alec was eighteen, he was caught standing over their mother's corpse with a stained statuette in his hand and blood all over him. He was exiled, and Bertrand refused to come with him, and so for the first time in his entire life he was alone. The next ten years alone were crucial in Alec's character development: he learned to be his own person in the face of a lot of trials, and emerged...honestly, more mature, capable, and self-possessed than Bertrand had ever been.

And so comes the final phase of their relationship: the rift. The two years Alec spent with Bertrand after being reitnroduced to English high society at age twenty-eight. Alec had changed and grown in ways Bertrand refused to acknowledge, wanting to still be the big brother and the dominant one in the relationship. With Bertrand patronizing him and taking him for granted, Alec only grew more resentful that Bertrand hadn't come with him, and finally after an insult too far, Alec killed him. He considers himself freer for it.