Two siblings appear as owners of a house in the deed. One wants to sell and the other does not answer. The question comes up by itself: can one sign for both?
No, unless the other has authorized it. The civil codes of Mexico City and of the State of Mexico agree on two rules: no one may sell anything other than what they own, and no one may contract in another's name without being authorized by that person or by law [1] [2]. The law recognizes each co-owner as having full ownership of their aliquot share (parte alícuota), not of the whole house. That is why the sale of the entire house needs all the co-owners, each on their own behalf or through another legally authorized person. What follows refers to those two codes, which do not always say the same thing.
What each person can sell
Both codes define co-ownership with the same expression: what belongs to several persons belongs to them “pro-indiviso”, that is, in common and undivided [1] [2]. What the law recognizes for each co-owner is an aliquot share. As long as the house remains undivided, that share is not a particular bedroom or floor. As to its size, both codes presume that the shares are equal: Mexico City's, as long as the contrary is not proven; the State of Mexico's, absent proof or agreement to the contrary.
In Mexico City, article 950 says that every co-owner has full ownership of their aliquot share and that, consequently, they may dispose of it, assign it or mortgage it. It adds that the effect of that disposal or mortgage as against the other co-owners is limited to the portion allotted to them in the division, when the community ends [1]. In the State of Mexico, article 5.153 recognizes every co-owner's full ownership of their aliquot share, and article 5.160 prevents them from disposing of it to third parties without respecting the right of first refusal (derecho del tanto) [2].
Neither of those articles authorizes a co-owner to sell what belongs to the others. Nor is the relative who is handling the talks authorized merely because they are in charge of them: the authorization of each owner, or of the law, is required.
If you sell only your share: the right of first refusal
Both codes say that co-owners have the right of first refusal [1] [2].
In Mexico City, article 973 establishes that you may not dispose of your aliquot share to outsiders if another co-owner wishes to exercise that right. To that end you must notify the others, through a notary or judicially, of the sale you have agreed, so that within the following eight days they may exercise the right of first refusal. Once the eight days have passed, the right is lost. The same article sets the consequence of omitting the notice: “Until the notification has been made, the sale shall produce no legal effect whatsoever” [1].
In the State of Mexico, article 5.160 establishes that co-owners may not dispose of their aliquot share to third parties without respecting the right of first refusal. Notice to the others is given “through a Fedatario Público or judicially” (a Fedatario Público is a public attesting officer), also regarding the agreed sale and with the following eight days to exercise the right. Once that period has passed, it is lost. Article 5.161 adds that, when the sale has been completed, the preferred co-owner may bring the redemption action (acción de retracto) to be subrogated to the rights and obligations of the third-party buyer [2].
Each article names only those two means of notification, and a message in the family group chat is neither of them. Both speak of “the following eight days” without further detail, so ask your lawyer how they are counted before relying on a date.
On a sale between the co-owners themselves there is an isolated court criterion (tesis aislada) published in January 1994. The First Collegiate Court in Civil Matters of the First Circuit interpreted articles 973 and 2279 “of the Civil Code” (“del Código Civil”), the same numbers that today deal with the right of first refusal in Mexico City's code, and held that when buyer and seller are members of the co-ownership it is not necessary to make the transaction known to the other co-owners, because in it they are not entitled to the right of first refusal [3]. It is the isolated criterion of one court, not binding case law (jurisprudencia), and it does not mention the State of Mexico's code. Confirm with your lawyer whether it can be invoked in your case before omitting a notice.
If a co-owner does not want to sell
Here the two codes do not say exactly the same thing.
In Mexico City, article 939 provides that those who have legal ownership of a thing cannot be obliged to keep it undivided, except in cases where ownership is indivisible by the nature of the thing or by determination of the law. Article 940 provides that, if ownership is not divisible or the thing does not admit of convenient division, and the co-owners do not agree that it be allotted to one of them, it shall be sold and the price distributed among the interested parties [1].
In the State of Mexico, article 5.143 provides that the co-owners of an asset cannot be obliged to keep it undivided, except in the cases where the law so determines. Article 5.144 provides that, if ownership is not divisible or the asset does not admit of convenient division, it shall be sold respecting the right of first refusal and distributing the price among the interested parties [2]. That article does not mention allotment to one of the co-owners, which does appear in Mexico City's.
These articles do not say before whom the sale is requested, how it is processed or how long it takes. That has to be reviewed with a lawyer in each case.