Too bad I can't read a word of Japanese, so I have no idea what manga you're talking about.

do they fuck?

Well no. Their granny is given bless to them. It might be some kind joke. They prentend not interest about one another. Sometimes, they show jealousy about one another person's relationship. It's bond hard to describe. That's all. The main story is about their interaction with the spiritual world.

She's another cousin. She can also see spirits. She's in love with a ghost.

These birds are the guy's demon servants.

http://et-caetera.net/?page_id=38

interesting, downloading