As one Kiwi to another welcome to Encyclopedia Titanica.
Looking at Katie's biography in: The Irish Aboard Titanic: parents Thomas and Mary; 4 older siblings. John the eldest, Elizabeth who was 2 years younger, Rose 2 younger again, Thomas another two years younger again and then Katie herself who was 2 years younger than Thomas.
However, I am confused because under the heading: 1911 census: ".... Thomas (65), farmer, wife Mary (64), married 40 years, nine children, seven surviving, Thomas (23) Bridget (24), Kate (19) ....."
I am guessing that your Great Great Aunt survived? If not what about Mary Mullin?
Hope your search goes well,