I have discovered cracked rafters in my attic. It has been suggested to add sister rafters. What are they and what do I need to do? Are there any other alternatives to consider?

sistering is the most inexpensinve way to go here. It will also be able to support the existing load if properly fastened. All sistering is is taking another 2x8 or 2x6 which ever the size of the rafter is and attaching this via 16d framing nails and make it as long as you can, the longer the better, just make sure you can get it up into the attic, 8ft may be all you can get.

What nail pattern would you use?

If you could get 3 to 4 in a row and then say at least 1 row per foot
Much easier if you have a nail gun,