Marriage Registration documents

The marriage is over and the wife and the husband are happy. He can register his wife's name in the army as her wife and the required proof can be submitted. Unless otherwise there is a specific complaint to the police on this marriage there will not any problem. As per your question the marriage with the willingness of your sister and now today she is 23 years. So not to worry about registering the document in the office. You can go ahead.

As per your question, the view I get is that your family and your sister along with your brother in law are happy with the marriage. You just need the marriage registration to be done. As you have declared that the marriage was solemnized in 2011 when your sister was 16 years old and was docile for the marriage.

A marriage which has already been formalized can be registered either under the Hindu Marriage Act, 1955 (where both husband and wife are Hindus, Buddhists, Jains or Sikhs) or under the Special Marriage Act, 1954 (where either of husband/ wife are not Hindu, Buddhist, Jain or Sikh). The marriage can be registered at the office of Sub-Divisional Magistrate on any working day.

You need to register the marriage before you submit the documents to the army. Once the marriage is registered, they need not worry as it is a legal document which can be presented anywhere. As you sister is major now, and she stands firm with the decision, there is no harm is registering the marriage and making it legal and valid for all purposes.

Can you answer below question
Do your sister has her marriage certificate of marriage?
If yes then if government approves marriage at that time then military wont have problem. If no then it is time you should get marriage certificate. It is not necessary to have particular date or year to mention since only relatives and priest of temple knows date. Many people of past generation even don't have marriage certificate. So you need to get some witness , temple priest sign and authorized letter that marriage solemnized as per ritual on so and so date. After submitting details in registrar office you will get certificate within few days.