I think you're missing the point that she wants to feel that she is welcome in your home regardless of mobility. This doesn't likely have anything to do with the specific invite of friends as much as her learning about that brought this up for her.
Was her mobility an issue before you moved there? Myself, I wouldn't have picked a place to live that our family couldn't come visit due to lack of mobility. My dad was sick for a long time and had mobility issues, I picked first floor places so he could visit and when I bought myself a small house that had a couple steps to get in, I had the porch modified to be more accessible, when getting in and out of his own house was getting too hard I had a ramp built for him. I can replace the money, I couldn't replace extra time with him.
If this was my in-laws, I would have done the same thing.