Online Nikkah
To accept Islam is a matter of faith; it needs no formal acceptance from any authority. If she wants to declare it, she can; if she wants to keep it to herself, it is her choice. Marriage needs to be publicly declared so that a man and woman are now husband and wife. Documentation serves the same purpose. If being a Muslim makes a difference in documentation, then do it accordingly. Informing your relatives is your problem, for which you need to make a sensible and serious decision. For Asian people, extended family matters a lot, so make your decision accordingly. Hiding marriage from family causes many problems. You may take some more time to make a more mature decision.