When I raise my jib with the sister clip system I end up with 6 inches of slack in the forestay. When you tighten the mainsheet does this remove that extra slack in the forestay? It always worries me that the small line of the jib halyard has all that tension normally carried by the forestay
Am I rigging it correctly?
You are not rigging correctly. You're likely going to break either the halyard or the jib. You need to pre-load the forestay before tensioning the jib halyard. Use the main halyard to hoist the boom about 3 feet up off the rear crossbar and tie off the main halyard. Then pull in a moderate amount of tension on the mainsheet so it pre-loads the forestay. Then you can set the jib halyard tension.