The west London side said that Thorgan, a 19-year-old forward, will join their under-21 squad “to continue his pre-season development and a loan move is being considered”.
The teenager, a Belgium youth international, moved to the Ligue 2 club in northern France at the age of 14 and made his debut in the senior squad last season, going on to make 14 appearances in all.
His brother signed for Chelsea from Ligue 1 club Lille in June for £32 million ($49.6 million, 41 million euros).
“Should they both play for the Chelsea first team, the Hazards will join a list of previous Chelsea brothers — Ray and Graham Wilkins, Ron and Allan Harris, John and Peter Sillett and Chris and William Ferguson,” Chelsea said on its website.
Meanwhile, Czech midfielder Tomas Rosicky will be out for between six and eight weeks after undergoing another operation on an injured Achilles tendon, the Prague daily Sport reported on Wednesday.
“There was some swelling that we needed to get rid of,” the Arsenal player’s agent Pavel Paska was quoted as saying. “Tomas has already started physio.”
Rosicky, 31, will miss the start of the English Premier League season in mid-August and is also a doubt for the start of the Czechs’ World Cup 2014 qualifying campaign on September 8 against Denmark.
The playmaker suffered swelling of his Achilles during Euro 2012 in Poland and Ukraine. He played in the Czechs’ opening game against Russia that they lost 4-1 and the first half of the 2-1 victory against Greece.
But he was forced to sit out the remaining games.
Rosicky has scored 20 times in 87 appearances for the Czech Republic and has previously spent 20 months on the sidelines in 2008-9 after suffering a serious knee injury which forced him to miss Euro 2008 in Austria and Switzerland.