Short answer. Yes. Article 1038 of the Civil Code obliges a person incapable of succession who took possession of hereditary property anyway to return it, together with its accessions. They are also liable for all the fruits and rents they actually received or could have received through due diligence.
What the law says
Any person incapable of succession, who, disregarding the prohibition stated in the preceding articles, entered into the possession of the hereditary property, shall be obliged to return it together it its accessions.
Civil Code, Article 1038 — Restoring What Was Taken. Read the full provision →
Taking possession does not cure the disqualification
Article 1038 addresses exactly this scenario: any person incapable of succession, who, disregarding the prohibition stated in the preceding articles, entered into the possession of the hereditary property, shall be obliged to return it together it its accessions. Physically holding the property, or even acting as though the inheritance were theirs, does not turn a disqualified person into a rightful heir. The obligation to return the property exists precisely because they were never entitled to keep it in the first place.
The return covers more than the bare property
The duty to return is not limited to handing back the original property. Article 1038 requires it to be returned together it its accessions — meaning whatever has become attached to or forms part of it since. So a disqualified person cannot keep improvements or additions that came with holding the property in the meantime; those go back along with the property itself.
Fruits and rents are also on the hook
Article 1038 goes further than just the property and its accessions: the disqualified person shall be liable for all the fruits and rents he may have received, or could have received through the exercise of due diligence. This reaches income the person actually collected while wrongly in possession, and also income they could have collected had they managed the property with reasonable diligence, even if they did not actually bother to collect it.
What this means for pursuing the property back
If someone disqualified from inheriting has been sitting on estate property, the rightful heirs are not limited to recovering just the property as it stood at the time it was taken. Article 1038 supports a claim for the property with its accessions plus an accounting of fruits and rents, so it is worth documenting what income the property generated, or should have generated, during the time it was wrongly held.
Limits worth knowing before you pursue this
The action to have someone declared incapable and recover the inheritance must be brought within five years from when the disqualified person took possession, and any interested party may bring it. If that person already sold or dealt with the property before a court excluded them, the transaction stands as to a buyer who acted in good faith — the co-heirs' remedy there shifts to recovering damages from the disqualified heir rather than undoing the sale. The disqualified heir, once excluded, is not left with nothing either: they may still claim reimbursement for expenses they incurred preserving the property and enforce whatever genuine credits they hold against the estate.