Taylor Swift performs onstage duet with surprise guest…her ex's brother

Any bad blood between Taylor Swift and Joe Jonas must be a thing of the past. First she and Calvin Harris double dated with Joe and Gigi Hadid in London, and now Taylor, 25, is duetting with his kid brother.

Nick Jonas, 22, made a surprise appearance at Taylor's New Jersey concert to perform his hit 'Jealous' with Taylor…and Joe and Gigi were in the audience.

We're not sure which is stranger: performing live with your brother's ex (while he and his current girlfriend are watching), or singing your own song at someone else's concert.

Check it out above.

Taylor does 'Jealous' pretty convincingly. Should Calvin be concerned?