So THAT'S why her brother is so listless when he is older! lol
@Kaarme I would argue that babies don't know they need to get stronger, and thus don't bother to try (duh, they're infants) but she is clearly conscious at this point (and for who knows how long before this scene?) so she could have been purposely trying to be independent as quickly as she could.
I actually JUST read an isekai where the MC was an infant and doing muscle training to stand on her own.