I also dont think its rule..
In my case.,.. My mother is 34D and I am a 36D, so technically I am bigger than her. So I cant lend clothes from her.. but at least we the same shoe size.. I have lent shoes from her before and she lent from me as well.
My family genetics, I come from a family of big breasted women from my mothers side, mostly DD's. My mother mentioned my grandmother was a 36DD. I actually inherrited most of my features from my mothers side of the family.. I have their figure type (a real hourglass effect), with bigger breasts. I always looked like my mother and with the feminine effect of HRT, if you compare a photo of my mother (at my age) with one of me, we look very much alike.
My fathers side, all flat chested A or B cups.. so I got nothing from them.