I'm not sure about the captivity thing, but I do know that what were originally asserted to be 'alphas' were the breeding pairs. They were parents leading their children.
There was a very flawed study years ago where a biologist observed wolves in captivity and determined that they fight for dominance within the pack. Later, he realized that in a natural setting, a pack is a breeding pair and their pups, as you stated. A natural pack is much more cohesive and doesn't have a strict hierarchy, but the ridiculous myth of "alpha wolves" won't go away completely, despite the same biologist's efforts.