My relative will be having a haraam wedding reception in a hall after the Nikah in the Musjid.

Answer

The un-Islamic, haraam wedding reception is mal-oon (accursed). The people who will be attending the haraam function will also become mal

-oon. Satisfying their request is not permissible.

Our practise here in P.E. at our Musjid is that we do not allow a Nikah to be performed in the Musjid if that Nikah is to be followed by a haraam reception.

It is improper to attend even the Nikah in the Musjid when it is known that the people who have gathered for the Nikah are contemplating to commit fisq and fujoor whilst they are sitting even in the Musjid. They are making a mockery of the Deen for having the Nikah in the Musjid. They should have their Nikah in the accursed hall where they will be having their haraam function. You should never participate in even the Nikah. And, make known the reason for your absence. It is not permissible to obey the haraam wishes of parents. Politely refuse.