My four sisters and I got a Sister Tattoo. You can see it here:
http://www.mindspring.com/~amy.webb/tattoo.htm
As far as a design, we really have no idea what makes your wife or her sister tick. I'd say that it's up to them alone to decide what to get. I know that other people's ideas can be useful, but it's a bigger thrill if they come up with the concept themselves. At least I think so, anyway.
Once they design it and have it tattooed, make sure they take pictures & post the URL here. I'd be interested in seeing it.