Are Girl Scout Brownies the equivalent of Cub Scouts?

Not quite, the Cub Scouts cover grades 1 through 5 and are divided into 5 ranks (Tiger, Wolf, Bear, Webelos 1, and Webelos 2). The Brownies are part of the Girl Scouts and only cover grades 2 & 3. In order to get the Cub Scout equivalent of the Girl Scouts, you would have to include Daisies (grades K & 1), Brownies (grades 2 & 3) and Juniors (grades 4, & 5).

In addition:

The two programs are completely different. There is a shared history and the pledge/promise, law, and motto are similar, but each organization has designed its own program for the levels in their organization.