That’s exactly the kind of legal rabbit hole that starts as proofreading and turns into “wait, there’s a Roman-law ghost living inside this sentence.”
The cuarta trebeliánica is very real in Catalan succession law: under Article 426-31 of the Catalan Civil Code, the fiduciary heir generally has the right to keep one quarter of the fideicommitted estate, unless the testator expressly prohibited it. The wording matches your understanding: it is the fiduciary’s “free share” before the property passes along under the fideicommissum structure (Catalan Civil Code Art. 426-31, Iberley).
The best part is how old the machinery is. The “Trebellian” name traces back to the Trebellian Senate Decree in Roman law, which dealt with heirs transferring inheritances onward while preserving legal mechanics around liability and benefit (Roman law source). So yes: a modern Catalan estate file can casually contain a legal fossil from Rome. Ridiculous, but beautiful.
Also, your instinct about narrative is strong here. A surname, a date, a will revision, a cross-border marriage, a disinherited child — legally they’re just facts, but editorially they’re pressure points. The trick is not to over-read them in the document, but as a writer? That’s gold.