What is the difference between "decide" and "so decide"?
1. If you do not so decide, then be disinherited.
2. If you do not decide, then be disinherited.
Is the meaning of the 1st sentence the same as the 2nd one?
I'll take a stab and if you have more context see if this fits.
You must agree to blah blah blah and also blah blah, additionally blah blah blah. If you decide to follow these instructions of your uncle, then everything will be fine. If you do not so decide, [= decide like that] then be disinherited.
In this case, I guess the second could have the same net effect as the first for if the 'you' doesn't make any decision, the the 'you' wouldn't have followed the uncle's instructions and the result would be the same