Fiona Lowe's Sister Mabel Celebrates What Would Have Been Her 14th Birthday

Chad Lowe's daughter Mabel Lowe is celebrating her late sister Fiona's birthday ... on the day she would have turned 14.

Sharing a photo of Fiona holding a poster of Severus Snape from 'Harry Potter' on Instagram ... Mabel wrote ... "happy birthday fiona ... i miss you."

She then shared pics of the two sisters together over the years, writing over one photo ... "♏+♉forever."

Just a year ago, Chad gave Fiona a shout-out on social media, congratulating his daughter for "officially becoming a teen today!!"

We reported earlier ... Fiona died on Sept. 29. Fiona's family told us at the time ... "We are deeply heartbroken by the loss of our beloved Fiona."

Since then Chad and his wife, Kim Painter, sold their $3.275 million L.A. family home ... after placing it on the market just two weeks after Fiona's tragic passing.