I was wondering if any of you can help me out.

Which sentence is correct?
1. Jill and John are sister and brother.
2. Jill and John are brother and sister.
3. Jill and John are brothers and sisters.

Thanks for your help.

The first is the best. The second doesn't preserve the order as well, but is also correct. The third is awkward but understandable. In most situations, native speakers will use the first or "John and Jill are brother and sister."
Hi esc,
I would use the word "sibling" (a person's brother or sister):

Jill and John are siblings

Hope this helps
 CalifJim's reply was promoted to an answer.