@GracieGirl,
Hi, Gracie! The kind of depression you are talking about is called "postpartum depression." You can google it for more information, but basically, it's caused by the rapid shift in hormones after birth. It's important to know that only a small percentage of women have major problems with this, so there is no reason to expect that you or your sister would. Also, it is treatable with medication and therapy, and it doesn't last forever.
If your mother left your family because she was suffering from postpartum depression, she would likely have contacted you when the depression lifted. My best guess is that postpartum depression was only part of what made her decide to leave. This is obviously a painful subject for your father. Perhaps he will find it easier to talk to you about it when you are older, but perhaps not. Some wounds are too deep to ever completely heal. You will have to respect that. Are there other relatives who might be able to tell you more about your mother?