Gender of person who sings Bleach's eleventh opening (Anima Rossa)?
According to Wikipedia


  Porno Graffitti (ポルノグラフィティ Poruno Gurafiti), also known as Porno (ポルノ Poruno) is a Japanese male rock band from Innoshima, Hiroshima Prefecture, Japan.


Current and Past Members only lists 3 names and Wikipedia only has a link to the past member Masami Shiratama which says:


  After leaving the band in June 2004, he released the album Great Pleasure in December 2005.


Akihito Okano (who does vocals) and Haruichi Shindō (who does background vocals) have profiles on generasia and both lists to have wives. Considering that same-sex marriage is not legal in japan and their images on generasia look like men, I would conclude that your sister is correct and that it is a Male singing it.

If you are still in doubt, I checked to see if I could find a live performance of Anima Rossa on Youtube and here is the Link