Kenton made a good point. My reply was geared toward the probable cause and did not address any possible solutions.
Without seeing the entire attic and support framework I cannot suggest a fix for your problem over the internet. If the problem is related to the issues I listed there will be no cheap solution. Long term sagging of lumber also involves plastic deformation of the wood fibers which cannot be totally reversed.
Sistering on both sides of an already over spanned rafter only adds more dead load to an overloaded rafter. Ideally you probably will need some additional support at mid-span of the sagged rafters. There are a few ways of doing that but each method is customized bases upon your framing situation and interior wall support locations. Sagging over large span areas like a garage may require a support beam in the attic to support the ceiling joist and any diagonal braces installed to support the rafters.
My main point is you need to identify the root cause of the problem before you spend money trying to fix the symptom.