My late husband, who was permanently mobility impaired but not on a walker, and I traveled by long distance train many times, but only on single-level trains. (I have since traveled on Superliners so I'm familiar with their features.) In the course of everyday life and in our travels, he would occasionally need to navigate some stairs. He was able to do this with a good handrail and my help. I'm much smaller than he was, and no spring chicken, but I was able to assist him adequately in lots of things.
My main reason for posting is to point out that you will have your wife, brother, and SIL along, and to suggest that you consider how much help to you they may be. Every person's situation and limitations are different, but don't discount their assistance when making your decision about whether to take the trip. Taking a trip solo and taking a trip with three companions are two different animals!
As others have noted, if you could get the downstairs accessible unit and of course have your meals brought to you, you could totally avoid climbing the stairs. You couldn't go to the lounge, but I would not let that factor cause me to cancel a trip. On one of my scenic trips, the lounge was always full and my friend and I never got to use it. I could see the scenery just fine from my sleeper unit so it's not that big a deal. I also enjoy the dining car, but on my husband's and my final trip together, we had our meals brought to us, and again I wouldn't let that cause me to cancel a trip. Your views from your sleeper will be fine. In a bedroom, you can also see out the windows in the aisle on the other side of the train. Edit: I believe the Superliner accessible units span the width of the train, with perhaps slightly smaller windows than the bedroom units.
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If you could navigate the stairs upon boarding and again at your destination, you could remain on the upper level the rest of the trip. You would not have to leave your car or even your bedroom unless you choose to do so. The stairs and aisles are narrow, which is an advantage for some but not for others.
I will also add that Amtrak personnel were always very helpful to my husband and me. Crews can and do vary, however.
All in all, with three companions, I tend to have a positive take on your situation, but I don't know your physical limitations. Booking the accessible unit would be the "safe" choice, and might or might not be necessary for you. If you choose to do that, you should have no problem getting the special lower fare. My husband was never asked for any documentation, but it was pretty obvious he had mobility issues. We always brought a copy of our handicapped parking hangtag, but never needed it.
Best wishes to you!