No, from how I see it and understand it, Lision never acknowledged Lestia as his sister and tried his best to keep a distance from her. Lestia thought he should have been treated as family but slowly gave up on that idea after slowly being attracted to Lision and was always sad he kept his distance. That is until it was impossible for him not to be attracted to her and now wanting Lestia for himself.
But they were never siblings? And the actual children of Veschester never truly thought of him as a family member. So how is that weird when he was only there for protection and under a deep cover to stay alive? Lision was not a true member of that family, therefore, not weird that the two fell in love sense they were raised to be family.
It was only a cover, he was never truly her brother. What part are you not getting? They weren't raised to be siblings at all, he never saw Lestia as a sister and she never saw him as a brother. Just a child who was brought into the family to act as member until safety can inherit tue Dukedom and then throne.
They were never siblings to begin with.
This isn’t some weird one when she ends up fall for someone who she thought was family or who practically brought her up right?