You have the right to file the Congressional. However, if you can have the Brigade Commander who imposed the punishment or reprimand write a clarifying letter, stating he did not intend to take you off the promotion list it would be more helpful to your cause.
I concur with Attorney Pardue, try to resolve this internally first. You have a right to contact your congressman (i.e. "file a Congressional") for whatever reason you feel appropriate. However, the likelihood of getting any resolution from a "Congressional" is relatively slim. Typically, when a military member makes a complaint to his congressman, the congressman's staff will read it, determine what agency needs to respond and then forwards the complaint to the agency, in this case the Army, attached to a letter signed by the congressman. The congressman rarely, if ever, takes sides. In other words, he or she will not become your advocate. Rather, they simply ask the agency to forward a reply to the complaint to their office. Once received, the congressman will write you a nice letter thanking you for your service and telling you that the agency response to your complaint is attached. Unless there is some political reason for the congressman to get involved, your complaint will be handled by staffers and responded to with form letters. Bottom-line, your best hope for relief is to advocate for yourself with your chain of command...respectfully, of course.
Answers to posted questions are for general interest only and do not constitute legal advice. No attorney-client relationship is established by virtue of any answer posted by the attorney.
I concur with the advice given in the previous answers. In trying to resolve this problem locally, you should avail yourself of the Army's open door policy, as established within your command. You can reference Army Regulation 600—20, Personnel—General, Army Command Policy available online at http://www.apd.army.mil/pdffiles/r600_20.pdf The excerpt below is from the Regulation.
2—2. Open door policies
Commanders will establish an open door policy within their commands. Soldiers are responsible to ensure that the commander is made aware of problems that affect discipline, morale, and mission effectiveness; and an open door policy allows members of the command to present facts, concerns, and problems of a personal or professional nature or other issues that the Soldier has been unable to resolve. The timing, conduct, and specific procedures of the open door policy are determined by the commander. He or she is responsible for ensuring that Soldiers are aware of the command’s open door policy.
You have grounds to file the Congressional, the question is whether or not based on the facts of your situation, a Congressional Investigation is likely to lead to you being reinstated to the selection list. This is dependent on the facts of your situation, if your chain of command is treating you inappropriately in light of what really occurred between you and the Junior Enlisted Soldier, if anything even did occur. If there are clear facts exhibiting some degree of fraternization, then the Congressional may not help you and only get you in more hot water with your unit. From what I have seen with Congressionals, they typically do not result in any real change, but they will get your unit's attention. If you have no need to stay under the radar, then file the Congressional. Based on your question, it sounds as though the letter was intended to be filed in your local file as opposed to being placed in your restrictive fiche. A LOR in your restrictive fiche can result in hindering promotion opportunities. If the LOR was filed in your local file, then once you PCS it simply disappears and you don't have to deal with it again. If your BC has an open-door policy, go speak with him or her. Emphasize your accomplishments, and ask your BC to reconsider placing the LOR in your local file. You will likely go much farther with this course of action than by dropping the Congressional bomb.