You should first find out how you feel about her and how she feels about you. It's normal for her to be afraid of her mother's misunderstanding before you decide on your feelings. After all, she is only 18, 9 years old. I think your relationship is more like friendship. She regards you as a brother and doesn't want to owe you. I don't think you two are suitable. After all, you should find a girl close to your age.