I've never made crab apple jelly, but I do make all of our jam and jelly. Are you planning to use commercial pectin, or make the jelly from scratch without? From what I understand, crab apples have a lot of pectin in their skin, so you should be able to get it to gel fairly easily without purchased pectin.
Are you planning to can the jelly jars when you're done? I really like this site for canning resources, and here are their crab apple jelly instructions:http://www.uga.edu/nchfp/how/can_07/...ple_jelly.html