Combine 2 cups flour, brown sugar, baking powder, and salt in a bowl. With a pastry blender, cut in the margarine until the mixture resembles coarse crumbs. Stir in the evaporated milk and beaten egg.
Press the dough into the bottom and partly up the sides of a 9x13-inch baking dish. For a thicker crust, press the dough into the bottom of the pan only.
Preheat an oven to 350 degrees F (175 degrees C).
Mix the rhubarb, gelatin mix, and 1/2 cup of white sugar.
Spread the fruit over the dough.
Combine the remaining 1/2 cup white sugar, 1 cup of flour, and softened butter in a bowl.
Cut in the butter until the mixture resembles coarse crumbs. Top the rhubarb with the streusel mixture.
Bake in the preheated oven until the topping is golden brown and the filling is bubbly, about 45 minutes.
