Centre-back Martin joined Rovers on transfer deadline day in 2006, and now winger Marcus has followed suit.
Blackburn's official website has confirmed Marcus has joined the Ewood Park club on a free transfer from Swedish side Halmstad, signing a two-and-a-half-year deal through to the summer of 2014.
The 23-year-old, who can also play up front, is now awaiting international clearance on the deal from the Swedish FA and FIFA. The Olsson brothers both started their careers at local side Hogaborgs BK and had a trial with Blackburn at the end of 2005.
However, while Martin signed a contract under then manager Mark Hughes, Marcus returned to Sweden and eventually signing for Halmstad in 2008, scoring 17 goals in 139 appearances.
Marcus has also followed his brother into the international side, earning his first caps in friendlies earlier this month against Bahrain and Qatar.
Olsson is Kean's second capture of the transfer window following the recent loan signing of Bordeaux striker Anthony Modeste.
And Kean is poised to make it a hat-trick of new faces later today with a deal for QPR full-back Bradley Orr.
The 29-year-old, who joined Rangers 18 months ago from Bristol City, has made just six Barclays Premier League appearances for the club following their promotion to the top flight last year.
Orr is understood to have already agreed terms and is currently undergoing a medical.