The Nashville alum, 34, honored the actor on what would've be his 29th birthday on Sept. 25, sharing a childhood photo of the pair sitting underneath the Christmas tree.
"Happy Birthday little brother," she captioned the throwback image. "I miss you everyday. I'll love you forever."
The heartbreaking tribute comes seven months after Jansen's passing. In a past family statement to E! News, Hayden and her parents, Lesley Vogel and Skip Panettiere, said that the medical examiner determined the artist died "due to cardiomegaly (enlarged heart), coupled with aortic valve complications."
"Jansen's heart could be seen in his eyes, and his charm in his brilliant, engaging smile; his soul in his masterful and revealing paintings, and the joy of life in his dry wit," the Feb. 27 statement read. "His charisma, warmth, compassion for others, and his creative spirit will live forever in our hearts and in the hearts of all whom he encountered."
Calling his death an "unthinkable loss," the family added at the time, "We love you so much Jansen and you will be in our hearts forever."
Like his sister, Jansen broke out into Hollywood at a young age. His earliest gigs included a 2002 stint on Disney Channel's Even Stevens, before going on to appear in TV shows such as Hope & Faith, Everybody Hates Chris and The Walking Dead.
Also similar to Hayden, who voiced Princess Dot in 1998's A Bug's Life when she was a kid, Jansen was a voice actor. His voiceover credits included the Jacob Two-Two animated series, Blue's Clues, Robots, Ice Age: The Meltdown and The X's.
Weeks before his death, Jansen shared a glimpse into his close bond with Hayden by posting a photo of the Heroes actress trimming his locks.
He quipped in the Jan. 24 Instagram post, "Not the first haircut she's tried to give me."