This is something that we don't like to think of, but there are some people who will never understand. Goldfish and bettas have always been kept in bowls, and you can cram ten fish and a pleco in a twenty gallon tank. It's the way things have always been done. The pleco lived for over a year, so it was successful, right?
The people at my brother's day program are like this. They just don't understand how a fish could require so much care. They had a pleco that I was trying to rescue. It died about an hour before I was going to pick it up. I had explained that the really big fish don't do well in a 10g, and not to listen to the idiot at the petstore because he doesn't know anything about fish, and I would go out with my own money and buy some fish to stock the tank with.
Two weeks ago, I show up and there's a goldfish in the tank, swimming alongside the two black skirt tetras. I wanted to cry.
The idea is "they normally live in little bowls, so it should be really happy here."
Like I said, some people don't want to learn. The creatures are "just fish", not like puppies or anything, so they don't need a lot of work or care.
Hopefully your mother in law isn't one of these people and can gently be swayed.
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Or explain to her that she's basically doing that to those fish. Let her know that they're constantly putting out waste so the more room they have, the less concentrated that waste is. Let her know, they grow better and are more fun to watch when they have the room they need. Let her know that more room = less work to keep the water healthy.
