Translating legal document

[Pura]

Hi, I'm reading a pre-nuptial agreement in Spanish and there is a specific section I don't understand. Can anyone help me?
It reads : Asi lo dicen y otorgan ante mi, el Notario, despues de haberle advertido del derecho que tenian de hacerlo por si y al que renunciaron."
I understand most of it, but I think it's the latter part of this sentence that is confusing me. Thankyou !

[sosia]

A quick translation
Asi lo dicen y otorgan ante mi, el Notario,
So they say and consent in face of me, the Notary
despues de haberle advertido del derecho que tenian de hacerlo por si y al que renunciaron
afterwars I (the "Notario") have told them that they had a right but they have abdicated.

I have no more info about this "right", must be mentioned before.
In a a pre-nuptial agreement, usually it's a warning like "you both have agree, and in the moment I the Notary take this paper, what it's written may prevail, and you can't change your mind later"(one person, with the consent of both, of course)

saludos

[Pura]

Thankyou for your help "Sosia". It is greatly appreciated.