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"Middle" in difficulty between South and North Sister, this is a nice climb with a great approach - just about 5 miles and 1500 vertical feet to great camping at treeline (from the east). A bit of snow travel, gentle slopes, and one pitch of steep near the top make this a good route for those looking for a moderate multi day trip.
Be wary of crevasses on this route! Roping up on the Hayden Glacier is highly recommended. There are some really B I G crevasses on this benign looking glacier that you may not see at all until late season - or until you drop into one of them.
The North Ridge route can be approached from the east or west. East is via the Pole Creek trailhead and Hayden Glacier. From camp, head up towards the Hayden Glacier, and follow the ridge along the glacier to the saddle. Then climb up the North Ridge. On the North Ridge is loose rock and perhaps a 1 pitch 35+ degree snow field. Early season ascents are possible, but a GPS or solid compass skills are recommended if the approach trail is snow covered.
An alternative route from the West side is more of the NW Ridge, more of a boulder-hop staircase than the scree of the North ridge proper. Check with the USFS for permit requirements if you are coming in from the west via Obsidian Trailhead.