A close relative is getting married. After the Nikah there will be a haraam wedding reception in a hall.

Answer

If after the Nikah there will be an un-Islamic wedding function, then you should not attend even the Nikah even if it is held in a Musjid. Display your aversion for the haraam wedding reception with your abstention. Imaams of the Musaajid should not perform Nikahs which will be followed by haraam wedding receptions. Let the evil and flagrant transgressors get hold of some mercenary, bid'ati, qabar-puja (grave-worshipping) molvi/sheikh to perform the nikah in the haraam hall where the haraam wedding reception will be held under Divine Curse.