It looks like Luigi will be joining his brother in the world of Lego Super Mario soon enough, as a listing for a Luigi set has been spotted on Amazon China.
The since-removed listing was spotted by Lego fan site The Brick Fan (in turn spotted via Eurogamer), which describes a new "Adventures with Luigi" set that will allow owners of the Lego Super Mario set to expand their block collection with a new set featuring the green-suited plumber, who'll have similar features to Mario and allow you to connect your sets to him.
Funnily enough, it appears to be Lego themselves who first sowed the initial seeds that Luigi might be on the way to the sets, when those downloading a recent firmware update for the Mario figure in last year's original Lego Super Mario set noted that Mario now cried out for his brother. Lego acknowledged this behaviour in a Facebook comment to a customer whose child had made their own "Luigi figure" from other bricks, saying "We did notice that LEGO Mario has started calling for him, we're looking into it and hope to have clarity on why this is soon. Stay tuned!"
At the time of writing, an official announcement from either Lego or Nintendo is yet to be made, although Brick Fanatics notes that the Amazon listing claimed a release date in this summer, so it seems we'll likely hear confirmation one way or the other whether Luigi is about to debut in Lego Super Mario very soon.