Is there a yuri manga where a girl is friends with a pair of siblings, a brother and a sister, and starts going out with the brother, but then realizes that the one she likes is the sister?

With brothers I only know of a few oneshots:

"Hiyoko Densha wa Kyo mo Yuku" the fourth story in Sprout boy and seed girl - a oneshot from a shoujo collection so not as conclusive as you might like.

The female body

The tenth ten commandments

There are a few more I believe, I'll probably remember them later.