Is there any truth in the legend about the birth of Siddhartha?
It depends what you mean by truth. If you mean did this actually occur then I would agree it does seem far fetched. However if you understand it as a myth then it will have truth. I think sometimes we understand myths as been basically being synonymous with fiction. However myths have a deeper truth that is relevant to the culture or society that it comes from. Authors such as Joseph Campbell go further and say that myths have a truth that cuts across cultures and societies - a comparative mythology.

The story above could be understood as a variant of the hero story which typically have miraculous or supernatural beginnings. The story of Christ could be understood in the same way (though obviously not everyone would be happy with this interpretation). This kind of comparative mythology can be seen in other places in the Buddha's life story. His enlightenment experience can be seen as occurring on the world navel (axis mundi) as well as more prosaically under the Bodhi tree.

Joeseph Campbells book, The Hero with a 1000 Faces, is a good place to start if you are interested in these ideas.