Save yourself a bunch of time and money. Find an engineer for a couple of hundred $$ to figure out what the issues are, if any, or spend your money on a fix you are not sure will work.
There is no way anyone here can really help you over the posts you get. You need a fresh pair of eyes determining if you have old wood, decay, pests or if it is just a problem with missing support under the posts. Then no sistering will help and you will be spinning your wheels.
Seen it a hundred times, get an engineer to tell you what you need. Then you have documentation of what the fix was for when you sell your home.
Good luck and tell us how you make out.