The 22-year-old met up with the squad earlier this week after being named in the provisional 40-man squad by manager John Keister. This has now been trimmed to 28 men, an extension to the usual numbers has been made to mitigate any potential impacts of Covid cases, and Kanu has made the cut.
The wide man made his international debut last month as the Leone Stars were beaten by Comoros in Turkey. Kanu was a second-half substitute.
The tournament starts on January 9 and is scheduled to end on February 6.
Kanu is expected to miss four Posh Championship matches and the third round FA Cup tie at home to Bristol Rovers of League Two on Saturday, January 8.
Sierra Leone play group matches against Algeria (January 11), Ivory Coast (January 16) and Equatorial Guinea on January 20. It is only their third ever appearance at the tournament and their first since the 1996 edition in South Africa. They have never made it beyond the first round stage before.
Speaking ahead of the squad announcement, Posh boss Darren Ferguson said: “We expect Idris to get into the final squad and so does he.
“And the club wishes him all the best. It’s a fantastic opportunity for him to play in a fantastic competition.”