@Tori-chan:
That's definitely a possible way to read it—and some of the way their relationship is framed definitely echos the way certain authors like to dramatize "forbidden relationships". It's definitely at least suspicious.
On the other hand it seems roughly equally-likely to me that a lot of that appearance is just an inevitable byproduct of depicting an unhealthy, destructive and self-destructive relationship in which he is in turn both emotionally dependant upon the affection of, and abusively dismissive of the full personhood of (as in the titular "marionette"), his sister.
(And also, it's quite possible it's canonically the latter, but with the author
or artist being inclined to roll out some fan-service for readers that are positively inclined to the former notion, complicating things.)