The pair feature together for the first time in the movie version of Filth, the best-selling novel by Irvine Welsh.
McAvoy, 33, plays crazed policeman Bruce Robertson and Joy is a florist who he confronts when she becomes a murder suspect.
He said: “It was weird.
“I had to threaten her violently and ever so slightly sexually as well. That was well weird.”
McAvoy said he is proud of his sister, who is carving out an impressive reputation thanks to roles in The Angels’ Share and The Wee Man.
READ MORE: http://www.dailyrecord.co.uk/news/scottish-news/james-mcavoy-sister-co-star-irvine-1755511