Short answer. Partly. A condition not to remarry is generally treated as not written, but Article 874 makes an exception where the deceased spouse imposed it on the widow — as here. Even so it cannot touch her legitime, so at most it can affect what she was left beyond that minimum.
What the law says
An absolute condition not to contract a first or subsequent marriage shall be considered as not written unless such condition has been imposed on the widow or widower by the deceased spouse, or by the latter's ascendants or descendants.
Civil Code, Article 874 — Conditions About Marrying. Read the full provision →
The general rule and the exception that swallows your case
The Code's instinct is that people should not be paid to stay single, so a condition of that kind is considered as not written — the gift stands and the condition is struck out. But the exception is drawn precisely around the situation you describe. A deceased spouse may impose it on the widow or widower, and so may that spouse's ascendants or descendants. So the clause is not void merely because it conditions your mother's inheritance on her not remarrying. Your father was one of the few people the article permitted to write it.
Where it stops: the legitime
This is the limit that decides most of the practical question. Article 904 provides that a testator cannot deprive compulsory heirs of their legitime, and cannot impose upon it any burden, encumbrance, condition, or substitution of any kind whatsoever. Your mother is a compulsory heir, and her legitime as surviving spouse is hers unconditionally. A remarriage clause cannot reach it. It can only operate on whatever your father left her out of the free portion — anything above the minimum the law already secured for her.
The permitted alternative in the second sentence
The article then offers a route that is fully valid: a right of usufruct, an allowance, or some personal prestation may be given to any person for the time during which he or she should remain unmarried or in widowhood. The distinction is between taking property away on remarriage and giving a benefit that was only ever meant to last through widowhood — support while she needs it, or the use of the family home for as long as she is alone. Testators who want this outcome are usually better served by that structure than by a forfeiture clause.
What to read in the document
Work out first what your father actually left your mother and how it compares with her legitime, because if the gift was no more than her legitime the condition has nothing to bite on. Then look at the drafting: whether the clause takes property away on remarriage or grants a benefit limited to her widowhood, and whether it is aimed at a specific property or at her share generally. Remember too that her half of the conjugal or community property is not part of the estate at all and no clause in the will can condition it.
Related provisions
- Civil Code, Article 874 — Conditions About Marrying
- Civil Code, Article 904 — The Legitime Cannot Be Taken Away