Muggleborns have a distant magical relative, usually a squib who married a muggle. This is a quote from Harry Potter wiki:

>Muggle-borns inherit magic from a distant ancestor; they are descended from Squibs who have married Muggles and whose families had lost the knowledge of their wizarding legacy. The magic resurfaces unexpectedly many generations later.

It also says that magical siblings can be born as in the case of Collin and Denis Creavey, but not always for example Lily and Petunia. 

The explanation in the wiki comes, in part, from a [30 July 2007 webchat with JKR](https://web.archive.org/web/20080828113728/http://www.bloomsbury.com/harrypotter/default.aspx?sec=3).

>**J.K. Rowling:** Muggleborns will have a witch or wizard somewhere on their family tree, in some cases many, many generations back. The gene re-surfaces in some unexpected places.