It's a bit similar to "Synchrome" by Zero no Mono. Brother and sister feel the same sensations (i.e. if brother is aroused, sister feels it, etc.)
One morning sister goes to brother's classroom and calls him out, scolds him. (I think she was in the middle of a test and she couldn't concentrate because her brother was feeling sexually excited and because of their weird condition, the sister feels it). They have a brief angry dialogue and have sex in the school shortly after.
I really can't remember the title. Send help. Thanks in advance.