Good Morning, 

Thanks for your question. You are correct in thinking of leaving the stems and leaves in your garden. The insects and pollinator hibernate in the vegetation. 

Please click on the following link about overwintering insects: https://xerces.org/publications/fact-sheets/nesting-overwintering-habitat

Over the past fifty years, we've lost an amazing amount of pollinators and beneficial insects. The vegetation you leave in the garden will also start to decompose and nourish the soil. The birds also forage in the leaves and stems for food and insects. You may be able to educate your mother-in-law about the importance of leaving the vegetation in the garden over the winter. It's also a good idea to rake some of your leaves over the garden in the fall. The leaves are insulation and habitat for the insects. 

Here is another link about native plants: https://extension.umn.edu/about/our-stories/news/yard-and-garden-news/native-plants-for-minnesota-gardens

Here is another link about "soft landings" under trees for the insects and caterpillars: 

https://www.pollinatorsnativeplants.com/softlandings.html

Thank you for asking and protecting our pollinators. They need help!