There's really no need to put sisters in, and they are not a good way of repairing broken timbers as they make a localised hard spot, causing the neighbouring ribs to break (according to John Leather). Cut out the broken section of rib, and let in a section of new timber using a scarf joint. The new piece is prepared by laminating to the correct profile on a jig. Try and use the same timber as the original (usually oak or maybe elm). Glue the scarf joint with epoxy, don't glue the rib to the planking. Unless you have a friend who can help you with copper nails and roves, use silicon bronze woodscrews to re-fix the planking through the original nail holes.