tattikat2:
The elite lines are for all elites at SNA, and unlike some airports, all elite levels qualify. I've used 'em with my boarding pass (I've never pulled out my MP card), and brought my non-elite family with me. If you've got a non-elite BP you may have to finesse your way through.
OTOH, you may want to enquire about special needs pax - they may allow you to go to the front of the line, especially if you ask for a wheelchair for you mother-in-law, or if she appears frail. I've never had to deal with such issues, but I do know from experience that most airports are pretty good about those with infants.
Finally, I've not seen waits at SNA in excess of about 10 minutes since spring (afternoons), so it's possible that there isn't even an issue here.
Good luck!
-btr