The most common symbols used to indicate family are those for brothers, sisters, father and daughter relationships and the Celtic symbol motherhood. Apart from indicating the relationship many of these symbols were used as a way to worship or revere these figures.
The Arrow knot, or brotherhood of the arrow, is the most common representation for brother used today. While the symbol doesn't directly mean "brother" (it is more a symbol for the brothers of the arrow), it is commonly viewed as a symbol for it.
There are a few versions of this arrow knot, which you can see in the pictures below.
Clip art/pictures of the knot: http://www.allaboutcelticsymbols.com/eBook/InvisibleBoxImages/arrow.jpg
http://thumbs4.ebaystatic.com/m/mFxplOSR-JRXAuHzHDnsKGg/140.jpg
Tattoos: http://www.checkoutmyink.com/tattoos/lxadub3/brothers-of-the-arrow-knot
http://blog.blacklinestudio.ca/wp-content/uploads/2010/12/IMG_9790-300x200.jpg