"I have told them time and time again that I think good friends should call each other..." is a passive aggressive technique.  "Why do you ask mom why I haven't called? Why don't you just call me?  I would love to have you call me sometimes"  is a mature and direct technique.  She isn't "friends" with these 2, she's a relative - why would she pontificate about what "friends" should do?  

Also, is it possible that on the few occasions they have called her she has cut it short because she was busy, and they have no idea when to call her? The direct questions above would reveal this.