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Portugal — Reacquiring Lost Citizenship
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The claim here belongs to her, not to you — and that is good news, because hers is one of the simplest declarations Portuguese law offers.
A Portuguese woman who lost her nationality by marrying a foreigner under the law before 1981 reacquires it by declaration, with no opposition procedure. Crucially, the reacquisition is backdated to the date of the marriage. So once she has made it, she is treated as having been Portuguese when her children were born, and those children — whatever age they are now — can register as the children of a Portuguese mother.
That makes the order of operations everything. If she is alive, her declaration comes first and yours follows from it. If she is your mother, your own claim is then the strongest kind there is: child of a Portuguese parent, no residence, no test.
If she has died, the position is harder and worth proper advice: what matters is whether the nationality can be established as having been hers, and how the registry treats a line that was never registered in her lifetime. Do not assume it is hopeless, and do not assume it is straightforward.
Either way the first document to find is her Portuguese birth record, and then the marriage certificate that caused the loss. The central registry in Lisbon can search the register by name, date and place.