IIRC, he cheated on his wife with his cousin. I don't have web reference. I went to an Einstein exhibit (at the OMSI) that had a bunch of hand-written letters and such by him (in addition to attempting to make his theories more accessible). Though 'cheated' might be a little relative. I think that he may have just been spending a lot of time with her and writing her love-letters rather than just sleeping with her.

Here are some quick links I turned up on the exhibit:

http://eastpdxnews.com/general-news-features/explore-space-a...

http://www.flickr.com/groups/alberteinstein/pool/tags/omsi/

The Flickr link has a bunch of photos of the documents, but I don't none of them are labelled, so it may be a bit time-consuming to search for anything specific.