there is a part of junjou romantica that is called junjou terrorist (it's about a boy and his sister's ex-husband) but i don't know exactly which chapters are only jt.
anyway, if you can ignore a bit bad art at the beginning (too big yaoi hands lol) you should read the hole thing, it's really nice
http://www.mangago.me/read-manga/junjou_romantica/
Sister starts dating a guy that brother "knew" and fell in love with and didn't think he would see again but still a good story, some people don't like but I did. Its called Suki Ni Ikenai Hito by Cj Michalski...
Oh oh oh oh oh!!!! Kawaii Hito-Pure by Takaoka Mizumi!
About a boy and his sister ex husband or lover.
Does anyone know it? If you do please help me out.