Mom dealing with depression?

My mom was very depressed a few years ago, so we got a dog. She was two at the time, and now she is five, and my mom seems to be much happier ever since we've gotten our dog. However, my sister has already left for college, and I will be in a few years as well. Once I leave, my dog will have about 4-8 more years of life, but I'm just worried that my mom will become especially depressed, even more so than before. Neither me nor my sister will be home to take care of her, leaving only my dad. I just want to know how she will be after my dog passes, and what I could do to help.