All of this is explained subtly through the game's story.
Both characters are direct children of Alexander. Alexander desperately wanted a male son and went through great efforts to get one but failed to do so knowingly/officially, which is why he created the program and split up the company between his female children. Kane and Olivia were NOT known to publicly exist as Alexander's children and your characters' mothers purposefully decided to leave you out of that life, presumably to harden the characters but also keep them away from the company for reasons unknown. The story starts when the company finds all of you and immediately takes contact with you. He ordered Dominique to oversee it, and it's meant as a way to test Kane to make sure he's fit for the role and put him through hardship to make him stronger/make him prove himself. The sisters get very friendly with you when you start rising past K7, the program is just a way to test Kane and make sure he's fit to lead the company. It's not directly stated that he's the named successor but imo it's heavily implied that he's to inherit the central leadership role of the company, if he is capable of it.
Regarding Olivia sub route, the default name "Kane" given for the main character sounds phonetically identical to Cain, a character from the Bible who killed his own brother. This could be purely coincidental or it might, down the line, mean that only one of the children is to take power over the company and that (evil) Kane will have to betray his sister and bring her down in order to secure power for himself.
It's also to be noted that it's not implied that all sisters are complying with this. They regularly spy on eachother, and if you take the Olivia good route and spare your second brother one of the sisters will take advantage of you and try to put you down, implying that she wants you to help HER take over the company instead and keep you in a background role beneath her, excercising her power on you by doing things to the second brother against your wishes.