@Kevadu
I don't agree with you and agree with people that think it's incest. There are 2 main points for why most people think it's incest. It's because of her face when she saw them holding hands. It gives off the feelings of a girl in love being shocked by them being together, not sure the face of a sister discovering her brother is with a woman.
The 2nd point is that she became depressed/had empty eyes/thrown away the gift. Again it show that her feelings for her brother are really big.
= Sister in love with the brother.
Or she is bro-con but even then is borderline incest feelings.
If she was just a normal sister she would just be depressed, possibly comedically drawn with some back lines, and not soul crushed like that in her bed even though she recovered in 1 night.