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.
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.