The proposed disclaimer may not have the intended effect, so you do need specialist legal advice on that.
Your wording is incorrect and for a disclaimer to have effect, the person disclaiming needs independent legal advice.
Qualifying for agricultural relief is not a given, however you have a potential period of time here to review the tax situation before the probate issues ( assuming your brother has not gone into occupation- a definite no no at this point). He should immediately take advice to ensure that if he doesn't now qualify, he may be in a position to make himself qualify.
You and your mother need advice on your intentions and how your brother can potentially pay consideration to you with good rather than bad tax consequences.
In short, I think this is an estate where you certainly need advice and to try to take out a personal grant now could have very serious tax consequences, not to mention legal consequences which are not necessary.
A good experienced probate solicitor will guide you through all the potential pitfalls and is well worth the cost of their fees.