Have a conversation with your sibling and find out their true feelings. If you feel that you can approach your parents without putting yourself or your sibling at risk, initiate a conversation with them. If not, find a trustworthy third party to help you address the situation with your parents.
You may also want to seek legal advice, and counselling from someone with experience dealing with cases of forced marriage.
If your sibling is underage (under 16 years old), the situation must be reported to the Children’s Aid Society. This organization’s main purpose is to protect children.