The in In a nutshell answer answer has to be legs and knees. If your best years are behind you like me perhaps you will remember fondly the days when you used to refer to your knees as left and right rather than good or bad.
Thankfully there is a lot you can do cheaply and without taking up too much time to prepare yourself for a fun safer trip to Niseko and hopefully give you a few more extra hours on the slopes each day. The stronger your legs are the more protection the muscles can offer the joints and the less likely you are to develop a level of fatigue where you can no longer provide a safe level of stability.
The internet has more knee exercises than videos of cats but what you are looking for are exercises that recruit multiple joints at once and also have a slight element of instability in them so that you are working not only your muscles but also balance systems and your proprioceptive systems.
It’s worth starting with simple body weight lunge step and then moving onto more weighted options.
A Goblet hold on kettle bell is one option but some 5Kg-10Kg weights held either side are also good option.
Once you have achieved some fundamental strenght and stability you want to start add more plyometric dynamic exercises.