But despite her sister's cancer turning a corner with chemotherapy, Catriona says she knows the journey ahead will be full of challenges.
"It was very destabilising," Catriona admitted when reflecting upon the moment she found out about Lucinda's Stage 4 Cancer diagnosis.
"So often I look at my older sister as a maternal kind of figure," she said.
"You also feel ticked off, she's the nice one, she always puts herself last so you think, 'Why does this have to happen to her?'"
In April that same year, Catriona took to her Instagram to share the news of Lucinda's diagnosis, posting two pictures - one of her and her sister and one of her entire family - to accompany the caption.
“Hello, a bit of a personal post for me, today is my big sister Lucinda’s birthday, Lucinda is a true gem. Unlike me, growing up Lucinda never caused my parents an ounce of worry (other than liking AC/DC) I’d do anything for her blonde beauty (I remember Dad would cut her hair for years, standing on the laundry step with a ruler placed on her back) her smile and laugh warms everyone’s hearts,” Catriona wrote at the time.
The Getaway host then shared how Lucinda met her husband Johnno as well as how Lucinda became a teacher.
“Lucinda had no boyfriend dramas as she married her first boyfriend! Yep, at 14 Johnno spotted her across a room and said to his mate ‘I’m going to marry that girl!’, can you believe that and they’re still madly in love. They travelled the world together, then settled down to raise 4 cheeky children, who we love more than words can say,” Catriona continued.
“Lucinda and Johnno once had a children’s gift shop they were so talented at buying for little ones. Lucinda just adores children and is now a hugely successful, beloved teacher and we can never have too many of those right?”
WATCH NOW: Behind the scenes of Catriona Rowntree's Good Health photoshoot. Article continues after video.
At this point, Catriona delved into the heartbreaking news.
“I think you know where this post is going, yep, we have recently found out that Lucinda has Stage 4 Cancer, which is apparently incurable. I’m trying so hard to choose my words carefully here, other than to say a lot of tears have been shed, BUT Lucinda is fighting and positive. We all are. She has started chemo (sheer hell) and immunotherapy (our bright light) at #royalnorthshorehospital.”
“Lucinda is now unable to work as she commits to the fight of her life. Her girlfriends have created a #gofundme page to ease the household stress and I’ve asked her permission to share this with you. Lucinda is the first to be placed on a new medical trial that may one day help a loved one of yours.”
“So, if you’d like to donate, thank you, trust me when I say to live your life to the absolute max, none of us know what each moment holds for us and please say a little prayer for Lucinda tonight. Thank you 😘,” Catriona concluded before adding a link to the aforementioned Go Fund Me page.
Fans instantly flooded the comment section of Catriona’s post and offered their thoughts, prayers, and well-wishes to both Lucinda and Catriona.