Stripes at Corner Seams

  There are probably numerous thoughts about and ways to join the stripes at the Inside Back (IB) to the Inside Arm (IA) corner, and each of them may have merit. This is how I'd probably do it.

    You will want to center all the stripes at the center of the sofa, and then match the stripes outwards from the center.

Sew the Seat and IB Together First
     To get the stripes to fit correctly at the IA-IB corner, I'd fit and sew the Seat and Inside Back (IB) before I did the IA. On this type of sofa, you could probably seam the seats at either the center or at the legs. If if was a plain or all-over pattern fabric, I'd join it at the legs. However, since this is a stripe, I would suggest you put one seam in the center of the IB and seat top. (Cut 2 full width fabric pieces for both the seat and the IB.) When you sew the fabric together in the middle, take care to put the center seam at the edge of the stripes. You should be able to sew the fabric panels of both the IB and seat so that the repeat pattern of  the stripes continues and looks consistent  from one side of the seams to the other. 
     After sewing, then go ahead and attach the seat fabric to the sofa. When centering the seat, don’t use the seam as the center, but use the stripe next to the seam as the center. This will allow the stripes at the IB-IA corners to match on both sides of the sofa.

Then Join the Arms
   At this point you should have the two halves of the IB sewed together, use a couple staples to tack the fabric in place (so that the IB fabric doesn't move) while you are fitting the IB. (Line up the center carefully before tacking, and use the same center as you did on the seat, the stripe next to the seam)
   To fit the IA to the IB, I would suggest that you find the center of the curve (where the inside back (IB) meets the inside arm (IA) from top to the bottom and then reposition the seam so that it follows the center of the curve. Then line up your stripes so that the stripes on both the IA & IB look the same. You will need to fold seam edge of  the IA fabric enough so that it matches the IB stripes. This is the way I’d match the stripes at the IA-IB corner: