If a mother is undergoing depression but rarely show signs of it, how can the family members help her?

Reach out to her. If you feel that she's having a hard time expressing her feelings, the family members should be the one to initiate. You may not want to ask her directly at first. Instead, let her feel that you are all there for her as a family. Give her all the security and love she needs while she's going through this stage. Later on, you may ask her to open up to you so you could help her further.