Rita,
See my post in Coupon Connection for Northwest PD937 zone certs:
http://www.flyertalk.com/forum/showthread.php?t=347582
This is not a companion fare deal but rather a discount travel coupon deal. The fare from MA to NM will be either $248 or $288 depending on the day of the week that you travel. You and your sister can get either CO, DL, or NW miles on these tickets.
If you can make use of them I am happy to send you three of them.