Siblings are linked roles, specifically a variation on Masons. Siblings are similar to lovers in that if one dies, typically the other will die with them shortly thereafter. Much like lovers, siblings are most often Town-Mafia pairs, however typically the town-sibling will be a Cop or other power role. Thus, the town sibling does not want to claim sibling, since lynching the mafia sibling would result in the town losing their cop. Likewise the mafia don't want to nightkill the cop, since that would mean losing one of their own.
Additionally, since siblings can talk OOT, there is often a game within a game, with the cop interrogating the mafia for who their scumbuddies are, while the mafia does the reverse to learn and/or mislead the cop's investigations.
On occasion, the siblings will actually have their own alignment (siblings-aligned), thus turning them into a third party. The typical win condition is to be the last ones standing, ala an SK. The combined knowledge of one sibling knowing the entire mafia, and the other being a power role, can be devastating. It's also very hard to balance; use third-party siblings with care.