Let me clear the air, no they're not siblings.
1.she clearly said she was an orphan.
2.the reason she's acting that way is being she doesn't want her grandfather to find out bc she thinks he'll be disappointed in her.
3.he has feelings for her and she for him but she thinks he only wants her for his body and she doesn't want to act upon her feelings as they're supposed to be siblings
ask me more:) and ps: I hate that bi*** (his mom)))