It is impermissible to accept the invitation to this banquet because it is a Bid`ah. It was authentically narrated that the Messenger of Allah (peace be upon him) said:
Whoever innovates something in this matter of ours [Islam] that is not part of it, will have it rejected.
When you refuse to attend, you are obeying Allah (Exalted be He) and you are not breaking the ties of kinship. Breaking the ties of kinship happens when you abandon the good and charitable deeds prescribed by Allah (Exalted be He) towards them. However, if you attend with the intention that you are able to change this evil act, (Part No. 9; Page No. 157) there will be no harm in doing so provided that you do not eat from the food served at the banquet.May Allah grant us success. May Peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!