Within a few minutes of picking up a guitar my fingers start to stiffen up, I crack them and then all is well, been doing this for 30 odd years now and the joints of my hands are some of the few which don't cause me problems at this point in my life. I never could crack my knuckles before I started playing guitar and if I stop playing for a few weeks, I cease being able to crack my knuckles.

These articles on knuckle cracking never seem to say anything but state, but sort of maybe could be but your aunt is almost certainly wrong unless or maybe in certain exceptions. For the majority of people I think we crack our knuckles for the same reason we often stretch when we get up, it relieves a tension that needs relieving.