Families are normally the last to show up at the reception.
If you have it restaurant style, people sit anywhere they want.
Guest will come in and sit at tables, if you know nobody at any tables you will start a new table. Most people will try to get as close as they can.
Family comes in, there no good seating left.
The reason why most weddings have seating charts is put people together.
Besides my father and his parents tables, we mixed our guest.
A lot of weddings do. We put people with together that we think would be find together.
Seating charts are very helpful.
You tell people Table 3 or 4 or 5, etc.
They have a place to go, it makes people going into a reception easier.
I would use a seating charts.
Just tell your mother in law that you will go with assign tables, people could pick their seats. Tell her she will get a table or two with important family/friends up front at the reception.