MC does not love her sister in a romantic manner. She just didn't know how to differentiate between the feelings since she's never known any other form of love, and her sister was the only one she had ever loved. Considering their background, MC probably got too reliant and attached and mistook that dependence on an older figure, as romantic interest. But when FL enters the picture and she experiences true romantic feelings for the first time, she is able to finally realise what she felt for her sister.
I am aware that the age gap is quite large in this manhwa, and to make matters worse they met when Shuraka was a child. However if you read the chapters you can clearly see that there was no sexual intentions from either side when Shuraka was a child, more so MC seems to actually hold parent-like affection during that time. Shuraka is the one who harbours romantic interest since the start.
Also clearly there is no sexual acts AT ALL. Some people need to realise that not all romance is sexual. Their dynamic when they met at first seemed very platonically romantic. They care deeply for each other but there's no romantic action between them that crosses the line to pedophilia.
Also I am pretty sure their entire romance is based on when they are both adults.
this is a well written explanation for those who still view this manhwa as problematic or concerning and additionally Tei was not fully aware that the arranged marriage will involve a child, and yeah I agree that their meeting before was only a filler (backstory) for the story to progress and their romance will blossom more in their adult years.
this, i feel like whoever read all that and went to red flag town is lowkey just making shit up