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