misconception about breastfeeding

So I made a post a while back about breastfeeding, I'm going to breastfeed when my baby is born, now my boyfriend is really misinformed and it's because of his mom who didn't breast feed any of her kids. She said how when you breastfeed you boobs sag quicker then when you don't but she didn't breastfeed so how would she know ?. I told her that's not true your boobs change regardless whether you breastfeed or not . My boyfriends sister had her baby recently she isn't breastfeeding because she believes her chest will sag . That's her choice but in a way their mom is trying to convince me not to ( sidenote: bf and I live with his sister their mom has been staying the last month every night cause of her having her baby and before) I talked to