re: When the Sun was born...
Dispersed cluster mates can often be identified because they have parallel proper motions. A cluster star evaporates from its parent cluster(usually due to galactic tidal influences) in such a way that the stars continue to orbit the galaxy like a cloud of widely scattered buckshot. This is how we know that many of the stars in Ursa Major originated in the same open cluster. Many other such associations have been identified, but our sun is very old and I don't think any of its siblings can be located this way. They're probably all over the galaxy by now.