Other forms: alpinists
If you're a serious mountain climber, you can call yourself an alpinist. A leisurely hike won't earn you the title of alpinist. Climbing Mount Everest? That's another story.
As opposed to a casual climber of relatively low-altitude mountains, an alpinist is extremely serious about his or her climbing. If your sister is an alpinist, she probably sets her sights on extremely high mountains, like those in the Andes, the Himalayas, or the Alps. In fact, the Alps, which lie within the boundaries of Europe, are the inspiration for the word alpinist, which specifies a climber of high, long-ranging, and ice-covered mountain ranges like the Alps themselves.