Hi Nancy,
I was in the situation once where I was concerned about possible PPD in a mom I was doing a postpartum job with. I chose to express my concerns to her husband and that worked out well. I felt like being very direct with her would not go over well and that an indirect approach was needed in that situation. Another approach that you might try is if you get a few minutes to just chat with her (yes, I know, unlikely with twins) and ask her how she's feeling physically, how she's doing in terms of feeling overwhelmed, does she know what physical and emotional signs she should discuss with her doctor, etc. Good luck to you in working with this new mom.
Mindy