We must truly become our brothers’ and sisters’ keepers.

Is this written correctly? OR should it be

We must truly become our borther's and sister's keepers. OR

We must truly become our brother and sisters' keepers.

Because the statement applies collectively to many people (we), the group to which it applies will presumably have many brothers and sisters. Hence the plural possessive use of the apostrophe (choice #1) is appropriate.

The sentence "We must truly become our brothers’ and sisters’ keepers" is written correctly. The use of the plural possessive form of both "brothers" and "sisters" with the apostrophes shows that it refers to the collective group of brothers and sisters possessed by multiple individuals in the group. This conveys the idea that everyone in the group should take responsibility for looking after one another.