The One Direction alum's older sister posted on Instagram in the aftermath of Liam's death ... paying tribute to the late pop star and revealing her initial reaction to the tragic news.

As Nicola put it ... she learned of Liam's passing through a news notification on her phone, which left her hoping the update was "untrue."

She added ... "I have spent days hoping that it was a mistake and somebody got it wrong. You were truly too good for this earth, you are an angel who just lived for making people smile and happy. You have always shown kindness to everyone you met and it shows with just how many people have confirmed what we always knew!"

As Nicola continued, she declared herself and the rest of the Payne family Liam's "biggest fans always and forever" ... praising her little brother for his "brilliant" talent and dedication.

Before signing off her note, Nicola promised to keep Liam's legacy alive, especially for the singer's son, Bear ... who he welcomed with Cheryl Cole in 2017.

She noted ... "We will make sure Bear knows about his daddy and how much you loved him, we promise this."

Nicola's tribute comes shortly after Liam's other big sister, Ruth Gibbins, posted to social media, in which she apologized for not being able to "save" the singer.

Countless others have honored Liam in the wake of his passing, too ... including Harry Styles, Zayn Malik, Louis Tomlinson, Niall Horan, Simon Cowell, Ed Sheeran and Rita Ora.

As TMZ previously reported, Liam fell to his death from a third-floor balcony at the CasaSur Palermo Hotel in Buenos Aires last Wednesday. The singer had been in Argentina to support his former 1D bandmate Niall's concert at the Movistar Arena earlier in the month ... ultimately choosing to extend his visit.

Police have said Liam jumped from his hotel balcony ... and that a possibly hallucinogenic drug, called "cristal," may be to blame. However, police are waiting for final test results for substances found in Liam's hotel room before confirming. The investigation is ongoing.