Part of it depends on your motivation in wanting her to do this. Does it irk you to see her sitting around idle? If she works, keeps house, laundry, meals, everything running smoothly, maybe "just sitting" is her downtime. (and - imho - your son should be helping with all that)
If it is because you want to share a special joy/hobby with her, approach it like that. That you want to be able to share ideas, patterns, love of fabric, the mystery of how it all comes together into a thing of beauty with her, that you can marvel on it together, a way to bond with her, then tell her that.
But start way simple. Maybe a couple hours together. Rotary cut blocks. Lay the plain blocks on a design table, show her 3 or 4 ways you put them together, how different the look can be. & sew a throw that is hers when you're done.
If she doesn't love it, leave it alone after you've tried.
I see QuiltE & I said pretty much same thing, posting at same time, sorry.
One more thing, maybe the "doing nothing" is just TV couch time with your son. They're in love & the draw to be close to your special person is a powerful magnet. :D