What should I do, as I fell in love with a girl and am waiting for an opportunity to propose to her, then I later learned that my younger brother also loves her?

Your marriage to the religious girl mentioned is permissible, and even recommended. The fact that your brother loved her does not prevent you from proposing to her, as long as he has not proposed to her himself. However, if your marriage to her would cause a greater harm, such as animosity between you and your brother, then we do not advise you to propose to her, in order to avoid enmity and hatred among relatives.