Short answer. Until that future date arrives, the legal heir — the person who would inherit by operation of law if your brother were not named — steps in to hold the property. The legal heir manages it during that interval but must give adequate security, with the participation of the instituted heir, before entering possession.
What the law says
the legal heir shall be considered as called to the succession until the arrival of the period or its expiration. But in the first case he shall not enter into possession of the property until after having given sufficient security, with the intervention of the instituted heir.
Civil Code, Article 885 — Designation of a Day or Time. Read the full provision →
How a time-based inheritance condition works
Article 885 of the Civil Code permits a testator to specify a future date on which a named heir's inheritance begins or ends. The designation is valid. The key consequence is that the property does not remain in a legal vacuum during the waiting period. Instead, the legal heir shall be considered as called to the succession until the arrival of the period. The legal heir is whoever would inherit under the rules of intestate succession if no will had been made — typically a parent, sibling, or other relative depending on the family composition.
The security requirement
Before the legal heir can actually take possession, Article 885 requires that he give sufficient security, and this must be done with the intervention of the instituted heir — meaning your brother has a role in determining whether the security is adequate. The security serves as a guarantee that the legal heir will surrender the property intact when the designated date arrives and the instituted heir becomes entitled to possession. The form of security — whether a surety bond, a mortgage, or another arrangement — is not fixed by the article and would typically be resolved by the parties or by the court supervising the estate proceedings.
What the legal heir may and may not do
During the interim period, the legal heir manages the property as a possessor called to the succession. This includes collecting fruits and income, maintaining the property, and acting on its behalf. What the legal heir generally cannot do is treat the property as permanently his — the obligation to deliver it when the period arrives is real and enforceable. If the legal heir impairs the property or fails to account for its income, the instituted heir has grounds to demand protection through the courts.
What this means for your family situation
If the will in your situation sets a future date for your brother to inherit, the first question is who qualifies as the legal heir in the interim. That depends on your family's composition and the order of intestate succession under the Civil Code. The second question is whether that interim heir has provided the security that the law requires. If he has not, your brother — as the instituted heir — has standing to demand it. These questions are best addressed during the probate proceedings for the estate, where the court can supervise the arrangement.