According to zap2it, the singer-actor has landed the role of a young Tommy Walker, played by Balthazar Getty.
Jonas has apparently signed up to star in two episodes which will be set in the past and shed some light on the relationships between different members of the Walker family.
Other famous faces rumoured to be in the frame for the flashback episodes include Fame's Kay Panabaker as Kitty, Anna Wood as Sarah, Kasey Campbell as Kevin, and Rachel Leigh Cook as Nora.
Brothers & Sisters continues Sundays at 10pm on ABC in the US and Thursdays at 10pm on More4 in the UK.
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