It would be a lie if Ayame loved Weiss with only sibling love and treated him as just a family member. It was far more than that.
Not just familial love, but the love between man and woman.
However, reality was disappointing.
As Weiss' sister, Ayame was confident in her looks and she was even slightly more beautiful than the Phoenix Imperial Princess.
But these very genes that made her look astounding were both a blessing and a curse. Blessing in the sense that she was very beautiful, but it was a curse since she became related with blood to her brother Weiss.
Even if she had thoughts that she shouldn't have towards Weiss, Ayame couldn't take that step since it was a point of no return.
Like Weiss, Ayame had both a high IQ and a high EQ. And because of that, not only did Weiss notice that she had thoughts about him, but the same went for Ayame seeing through Weiss' thoughts about her.