Final answer:
Without knowing the total number of Frankie's brothers, it's impossible to determine the probability that he will have to buy for one of his brothers for two straight years.
Step-by-step explanation:
The given information is insufficient to provide a precise answer to the question about the probability that Frankie will have to buy for one of his brothers for two straight years. To solve this probability question, we need the total number of Frankie's brothers or the specifics of the existing arrangement for selecting whom to buy for each year. If Frankie has four brothers, making them a group of five siblings, and if each sibling has an equal chance of being chosen each year independently of previous years, the probability would be (1/5) * (1/5) = 1/25. However, due to the lack of specifics, we can't determine the exact probability.