Hi Rick,
Will your wife also be attending the burial and luncheon? If so, then it is proper for you to attend with her. I would focus on being there as a way of showing your support for the family during this difficult time, and certainly your wife would appreciate you being there for her sake.
I understand the uneasiness in feeling like you are adding expense to the family by attending the luncheon (the cost person is usually not over $10 per person), but the priority in these situations is doing what is helpful or brings comfort to the grieving family. Since this is your wife’s side of the family, I would definitely recommend that you attend with her especially if she wants you there.
I hope this helps, and my condolences to your wife and her family on their loss.