Nikah is an open declaration of a contract by a man and a woman to live permanently as husband and wife. Therefore, the main objective is for the society to be aware of this fact that this couple has decided to live together as husband and wife. Including close relatives as witnesses and conducting this ceremony secretly do not fulfil the requirements of Nikah and hence, Nikah would not be valid. For Nikah to be valid, it needs to be announced either through societal channels or through legal channels. This is the bare minmum required for Nikah to be valid. Having a religious ceremony like a sermon etc. are very recommended acts but not part of the requirement.