Just noting that these types of above ground shed shelters aren't just "floating" on top of the concrete, but are bolted to the foundation and are tested for some pretty severe tornados. Perhaps not the very worst, but these are used extensively in our area. It's far superior to using an interior bathroom or closet imo, and way cheaper than digging into the ground.

So a great in between option. I also wanted something "in ground' because I've watched too many tornado movies. :) If you have unlimited budget, maybe? But we had half a dozen builders tell us that "in ground" shelters flood and cause all sorts of issues (even when placed in garages, which is where ours is going to be located). Is inside the garage a possible option for you? Anything else would likely require a structural renovation and possibly a permit.

My brother has a new in ground shelter and it is very cool. But if you or your mil have any issues with mobility, they won't be able to get in and out. The bolt-on sheds (some models) can work for wheelchairs too! My mom is in a wheelchair and will be living with us too, so that was a big selling point for me.