"She was hysterically crying, it was terrible," she said in court. "She was just terrified, bawling her eyes out."
A detective who dealt with the complainant that day said he could see she was "extremely tired and very emotional".
"She was on the verge of tears the entire time," he said.
The allegations followed an Oktoberfest function on the Army base.
Cook's defence lawyer, Roger Crowley, pointed out an inaccuracy in the sister's police statement compared to what she said in court, when the sister said the complainant told her she wasn't sure if she could call it rape, as there was no penile penetration.
The sister told Mr Crowley the complainant called it rape as she stood on the doorstep as well as throughout the explanation over the next 20 minutes.
Her police statement, however, says the complainant only said it later in the conversation.
Galvin's lawyer Chris Tennet questioned the sister's assessment of how drunk the complainant was.
The complainant said she was about five on a scale of one to 10 for intoxication while at the Unimog, and the sister said the complainant wasn't drunk by the time she reached her doorstep.
Mr Tennet pointed out the complainant's mother had described her as "pretty drunk" in the 111 call, but the sister disagreed.
"My mother doesn't drink, so I suppose anybody that drinks is drunk," she said.
The doctor who examined the complainant later that day said everything had appeared normal, although there was an area of redness and some sight swelling in the woman's genital area.
"It's very difficult to know from seeing a red area what's caused it," the doctor said.
"You can't tell whether this was consenting or non-consenting."
It was common for victims of sexual assault to have normal genital findings, she said.
A man whom the complainant had consensual sex with earlier that night was also called as a witness.
According to his police statement, he and the complainant left the party together, though she made comments about people watching, and was worried about gossip.
He said in his statement and in court that the complainant came to his barracks with him and left again not long after 3.45am.
He said he was certain of the time because he checked his watch while they were in bed.
Mr Tennet pointed out that a text sent by Galvin to Cook, saying he should make sure to get the complainant's number, was sent at 3.44am.
"That text would have been sent a minute before you checked your watch while you were lying in bed with [her]," he said.
Asked if he had a response to that, the witness said no.
The trial is expected to continue till tomorrow.
