。
If you're free, go help out with the branch family's work.
Literal
Free [conditional-なら] branch-family [possessive-の] work [object-を] help-[go-て来い].
分家 ('branch family') is part of the traditional Japanese family system, where the main household (本家) and its offshoots (分家) maintained distinct but connected identities. ~て来い is the imperative of ~てくる meaning 'go do X and come back.' 暇なら is blunt: 'if you've got nothing better to do,' implying the listener shouldn't be idle.