My sister just had a baby and is a little depressed. What can I do to help?

My older sister (28 years old) just had her first child a couple months ago. For the most part the child is healthy and happy. Her husband is a great guy and they have always seemed to be really happy with each other. Today I just found out from my mom that my sister is a little depressed, which I know happens sometimes after you have a kid.

I am 20 years old and am attending college a good 18 hours away from where my sister lives. My sister doesn't know I know she is depressed. I love my sister and want her to be happy. Any suggestions or ideas for what I can do to help her out? Or has anyone else experienced this before?

Thanks.