Short answer. Generally yes. Article 871 allows the institution of an heir to be made conditionally, or for a certain purpose or cause. Conditions like finishing college or caring for a sibling can validly attach to your inheritance, subject to other rules on what kinds of conditions the law will not enforce.
What the law says
The institution of an heir may be made conditionally, or for a certain purpose or cause.
Civil Code, Article 871 — Conditions and Terms Are Allowed. Read the full provision →
The Civil Code expressly allows conditional institution
Article 871 answers the basic question directly: the institution of an heir may be made conditionally, or for a certain purpose or cause. A testator is not limited to naming heirs outright with no strings attached. Your father is legally permitted to structure your inheritance around a condition, a stated purpose, or a specific cause he wants tied to what you receive, rather than handing it to you unconditionally the moment he dies.
Conditions, purposes, and causes are three distinct tools
The article names three separate bases a testator can use: a condition, a purpose, or a cause. A condition is a future and uncertain event the inheritance depends on — such as finishing college. A purpose or cause describes the reason or aim behind the institution, which can shape how it is understood even without functioning as a strict condition. Which of these your father actually used depends on how the will is worded, since each carries somewhat different legal consequences for how the bequest is interpreted and enforced.
Finishing college and caring for a sibling fit this framework
Both examples you raise — completing your college education and caring for a sibling — are the kind of concrete, ascertainable requirements that can function as conditions or as the stated purpose of an institution under Article 871. Nothing in this article singles out educational or caregiving conditions as improper; the article's language is broad enough to cover conditions like these as long as they are validly framed.
What this article does not resolve on its own
Article 871 establishes that conditional institution is allowed in principle, but it does not by itself settle every question about a particular condition's validity or how it operates in practice — other provisions elsewhere govern the effects of certain kinds of conditions and what happens if a condition becomes impossible to fulfill. Whether your specific condition is properly drafted, and what happens if you do not finish college or cannot ultimately care for your sibling, depends on reading the will's exact language against those other rules, not on this article in isolation. Getting that wording right at the drafting stage matters, since ambiguity over what a condition actually requires is often where disputes among heirs later arise.