EAD evidence help, thanks

I get the letter from NSC for my wife's EAD, They ask me submit the more evidence.
Here is the main point of letter.
"Submit evidence of the filing or approval along with the principle's alien registration number and documentation as to your relationship. Such evidence for determing relationship can be, but not limited to copy of a marriage certificates and/or birth certificates."
Can anybody tell me what document I can submit to INS to determing the relationship of my wife with me except the Marriage Certificate? I appreciate it very much.