Concept: Under the Transfer of Property Act, an interest may be vested or contingent. If enjoyment is postponed to a certain future event, the interest may still be vested. But if acquisition depends on an uncertain condition, the interest becomes contingent.
Step 1: Understand the transfer in Assertion.
The property is first given to the eldest sister for life. After her death, it is to go to the youngest brother only if he survives the eldest sister.
\[
\text{Youngest brother's right} \Rightarrow \text{Depends on survival}
\]
Step 2: Check Assertion (A).
Since the youngest brother must survive the eldest sister, his interest depends on an uncertain condition. Therefore, it cannot be treated as purely vested immediately after the death of the transferor.
\[
A = \text{Not correct}
\]
Step 3: Check Reason (R).
The reason states the general rule that when only enjoyment is postponed until a certain event, the interest may remain vested. This is a correct general principle.
\[
R = \text{Correct as a general principle}
\]
Therefore, the most appropriate option is that Assertion is not correct but Reason is correct.