At my moms funeral, my father in law was upset because he wasn't asked to sit with the family. I said just because his son is married to me does not make him (my father-in-law) a relative of my mother. If we had asked the relatives of my husband and the relatives of my brothers' and sisters' wives and husbands to sit with the family, practically the whole congregation would have been seated with the family. Would his son's being married to me make him a relative of my mother?

I've never been to a funeral where mourners chose, or were told to sit on, a side. That's the practice at a wedding. Isn't he family?

My understanding is that the immediate family sits in the first row, and every other person sits where they want.
But this should have been discussed with the clergy person beforehand. They also can 'direct' the seating so that family aren't blaming each other.

Your mom and his dad are in laws, but your dad in law should not be making such a subjective thing as seating an issue during a time of family bereavement.

I like Joys response.

When you have lost your mom the last thing you are thinking about is where people are sitting. I didn't care where I sat either so I plopped down on the second row. It was pretty insensitive for him to bring it up because at that moment it's not about him.