Since she was a child **
Yeeah stuttering can also be caused due to trauma, bu tin this case it seems to be something she's born with. However it can also be due to the way she was raised, most definitely neglect and degraded.
Neglected*
She was probably born with it and it got worst because of the way she was treated. If she was treated better and raised in the right way, her stuttering would be better by now or even better, gone. But dumbass father :/
The boomer dad didn't know the charms of the stutterer! But somehow I think she stuttered because she was too nervous since she can talk, I mean look how his father treat her all the time. If only she can be relaxed and be more confident, she might get better at talking too.