Pregnancy causes a noticeable change in breast size in most women, and the breasts will grow again when milk production is in full swing. As a woman tapers off the breastfeeding, the breast will decrease in size again, but not necessarily to prepregnant size.

It is interesting that your mom experienced the sagging and flattened breasts after breastfeeding but her sister did not. Chances are it still is related more to genetics, however. Sisters share a lot of genes, but don't necessarily have identical sets unless they are twins. Within any one family, one sister may be well endowed, and the other considerably smaller. And so it isn't unreasonable to expect that breast size changes might change from sister to sister. Most of these changes will happen regardless of whether a woman breastfeeds or not. It has been said to look at your grandmother's breast shape to get an idea of how yours will likely look in the years ahead.