#What can happen to a brother or sister with a weak conscience if those with an understanding of the true nature of idols are not careful in using their freedom?

A brother or sister with a weak conscience can be destroyed. [8:11]

#Who do we sin against when we knowingly cause a brother or sister in Christ to stumble because of their weak conscience?

We sin against the brother or sister that caused to stumble and we sin against Christ. [8:11-12]