Here is something that I didn't think I would say?

[Warpy]

16. Stairs aen't always a deal breaker

Stair lifts, that feature a seat that can flip out of the way when not in use have kept a lot of older folks in their homes instead of nursing homes. As your mom's mobility declines to the point she needs a wheel chair for long distances, you can install a temporary ramp over the front steps. There are work arounds for multi level houses. What I hope you realized you need is a toilet on both the ground and second floors.

Sorry big sis is an interfering control freak. At least she doesn't live close by.