If I make it the same length as the existing ones, it won't fit. There will be no way to install it w/o removing the roof plankings.

My only idea is to make it shorter and cut the piece that sits on the block
wall, cut the angle 1.5" higher. This will allow me to slide it in further,
then lift the ridge end and pull it into place. Then place a 2x4 below
the angle cut on the block wall side. Clear?

My jpg shows a drawing of existing rafter with the red line how Im thinking
of cutting the sister. It would be weaker, but this is all that I can think
of. If I do this I will attach it with screws and glue to the block wall top wood and bolt it to the existing rafter two bolts 1' apart at each end.

better ideas?

pics and my drawing: RAFTER SISTER Photos by paulsmars | Photobucket
please ignore the 1st incorrect jpg drawing. I can't figure out how to delete it. No wonder I stopped using photobucket a long time ago!

'Then place a 2x4 below
the angle cut on the block wall side.'

Sounds good to me in terms of how load is transferred down the wall. No weak cut sections hanging out over open space.

What about the sag? Im going to have to PUSH it flat with this new rafter. Will not the old sagging rafter push against the new, maybe sagging it too?

Three rafters where the roof sags have a measurable curve to them. 3rd one
Is totally destroyed by termites. Every place that I touch it, it crumbles.

Unless you get some thick boards up there to put under jacks to distribute load down the bearing walls, and jack up the roof without cracking ceilings below, and without disturbing the roof and popping nails, then I think you are just going to have to live with sagging rafters with sisters there just to add some support, and a solid surface to nail shingles to. Or replace the rafters and roof.
Call a builder for a visual opinion.