im 21 and my breasts still not fully developed

Hello i am about to become 21, and my breasts are not fully developed. I got my period quite late at 15, and never got any tenderness or signs that they were growing after that apart from the usual breast bud stage. Only just now have i started getting tenderness in my nipple and wondering if i am finally starting to develop fully? They do not have much fat around the nipple they are just like buds. My mum had B size breasts at my age i would be happy with even that, and now shes alot bigger and my sister started developing at age 15 and now shes a C so theres not typically tiny breasts in the family.