I really like this topic!
My Dad has always called me Piggy (while my big sis was 'Angel' hmmm..what does that say?!). Both parents call me Jen-wren and big sister always calls me 'Nif'. My real name is Jennie (vs Jennifer) and I used to get so upset at people calling me Jennifer so my big sis (naturally) decided this was a good way to torment me. This progressed to 'Niffer' and then 'Nif'. She still calls me this and is the only person to do so. It's funny that this name used to make me actually cry and now I'm so used to it (I'm 23). It worked really well for her when there was an advert over here for some air freshener with the slogan 'Gets rid of Nasty Nifs'!
Boyfriend calls me Jen-o or Jenster and a high school friend still calls me Jen-ball. No idea why. Like Amycat said, other people have picked up on this and tried to use it but it only works for her. I get quite annoyed if other people try to 'steal' her name for me (!). Most other folk just call me Jen (*yawn*)
I've always called my little sis 'Squirt' for the same reasons as my big sis...it used to make her cry instantly...aren't siblings terrible?!