In the case of the death of an elderly person in the family, to refuse the wedding banquet, you can take the following ways:

1. Apologize: Thank you for the invitation first, and then express that it is difficult for you to accept the invitation to celebrate the wedding banquet at this time, because of personal and family reasons, you need to deal with some important things.

2. Specific reasons: It can be mentioned that family members are experiencing grief and cannot participate in such celebrations. Generally speaking, people will understand and respect such decisions.

3. Comfort and blessing: Finally, I can express my condolences to the newlyweds and wish them a happy marriage. It can also make up for the regret of not attending in the future.

You can refer to the above to respond to the understanding of the inviter.