This is an often-discussed issue here: "I was also wondering about traveling by train/public transportation with luggage larger than a carry on. ...if we decide to stay in a different city for more that a few nights, I am sure they will want to bring their luggage."
Part of the issue with luggage larger than carry-on is that it's hard to get the pieces on and off the train in the time you have to board. Also, there will likely be stairs up to and down from (or vice versa) the platforms, not to mention the steep ones on and off the trains themselves. There may be elevators, but in my experience they are often not close to the platform I need, requiring long walks both directions.
Here's where the packing light advice comes in, along with the concept of each person being able to manage what they bring. I'm now 73. I use Schiphol as my connecting airport no matter where I go in Europe, with jet-lag recovery at the beginning of my trips and at least one night before I fly home in Amsterdam or Haarlem.
In 2017, I stayed in a B&B on an Amsterdam canal about a 10 minute walk from the train station. The stairs to my room were typical for a 17th century Dutch house, narrow triangular treads, steep risers and curved. The staircase itself was so narrow that I couldn't take my carry-on bag up or down with me and hold onto the rail at the same time. The owner took it up and brought it down.
I hate having to depend on someone else to do things like that, but the treads were too narrow to set the bag down, go up or down a few steps and repeat. You don't want to do that yourself, much less for 3 people.
This year I chose an historic hotel in Haarlem that had an elevator for my jet-lag hotel. Similarly, my last night hotel was close to the airport with a shuttle and an elevator, but I was on the ground floor so it wasn't really needed.
If you can find a hotel for the 3 of you, with an elevator, in a location you like and at a price that works for you, snap it up!
Now for some packing references for your mom and SO, accessible from or on this RS website.
Travel Tips: Packing Light.
Sarah Mudoch's Packing Light & Right video.
Travel Forum: Packing. Here's where they'll get lots of opinions on packing, not all of the same packing light type.