Ha the north facing Phildelphia garden... they are tricky I have one in its infancy as well.

I cant really reccomend specific plants as I dont really know all that much but if you sister goes to Greensgrow fams on Cumberland St http://www.greensgrow.org/(a really cool urban farm and nursery) they have a neat secton of plants that do well in shade and also a very helpful staff. I am sure they could help her put together a great garden.

Coverdale Farm, just across the bridge in Delaware, is having a native plant sale this weekend. You should bring a small sketch of your sister's yard, to the sale, and have the folks there help you put together a plan. Natives will do better in your sister's yard, and bring a bit of authentic wilderness to her city street.

Thanks to those who responded. Got an email from my sister this weekend and she went to the native plant sale that you recommended and got some plants. She's very excited about this project!
Thanks again.
Amy

She should still check out Greensgrow, they have terrific plants and helpful staff for continuing garden information. Also beautiful containers, herbs, and local veggies, cheeses and more. Also she should look into PHS membership and their workshops, some of which are free. Our local park has hosted several of these gardening workshops, many are geared towards small gardens and container gardening. I have a 17 x 13 patio garden facing southwest though, and a roofdeck garden facing northeast...kinda...