The romance author died on 19 September (15), aged 77, following a secret six-year battle with breast cancer.
Joan was left "completely devastated" as she only found out about her sister's health woes weeks before her death.
The Dynasty star has now honoured her sibling on what would have been her 78th birthday.
Joan shared an old black-and-white photo of herself as a child holding her baby sister on Twitter.com.
"On this day Jacqueline Jill Collins was born - a truly happy day for a grateful older sister. #JackieCollins (sic)", Joan wrote.
Following her sister's death, Joan put together a tribute essay for Britain's Daily Mail newspaper.
She wrote, "I don't think I will ever recover from the sadness of losing my beautiful baby sister... She will live on in the wonderful memories I have of her from our childhood and particularly from the last 15 years. I feel her spirit, I hear her wonderful laugh and I see her all the time in the hundreds of photos of her sprinkled around my home. She wasn't just a star - to me she was an entire galaxy."
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