Arbor Day Foundation is a good start (see first link below).

The Nature Conservancy, Rails to Trails and even some of our national parks have programs that offer tree planting "packages" for lack of a better term.

Across the U.S. there are a number of urban tree planting programs (Chicago's is a good example) that can involve your sister-in-law on a variety of levels.

Of the groups out there planting trees I'd steer clear of American Forest. Their end results for tree planting are... questionable.