The weather in the Cascades can be a little unstable this time of year- you never know what you are going to get. I tried it in May once and got fogged off.
http://www.splattski.com/2002/north_sister/index.htmlThen went back in July and did it under (mostly) clear skies.
http://www.splattski.com/2002/peak_week/north.htmlOn both tries, we parked at the Pole Creek TH outside Sisters. I don't know if you can get all the way in there this time of year. You might also ask on Cascadeclimbers.com
From the top of the South Sister (assuming you're going North-Middle-South), it's a long way back to the car (10 miles?), but do-able. For out July trip, we instead had someone pick us up on the Bend side. I think it's about 45-50 miles from TH to TH.