Ariana Grande's Brother Frankie Vows to Have Her Back 'Until the End of Time'

Ariana Grande's family continues to rally around her as she prepares to step away from the spotlight ... and now her brother Frankie is promising he'll have her back "until the end of time."

Frankie Grande shared a heartfelt tribute Friday after seeing Ariana's "Eternal Sunshine" tour ... calling the show a "beautiful celebration" of her artistry and saying he's incredibly proud of his little sis.

He added, "Family forever. I will have your back until the end of time" ... and said he was grateful he got to see Ariana "shine on this tour one last time."

The message comes as Ariana prepares for a major step back from public life. As we reported, she'll take a break from public-facing work after the tour wraps September 1 ... and she's already pulled out of a planned London stage revival of "Sunday in the Park with George" with her "Wicked" costar Jonathan Bailey.

Ariana later clarified the break was planned well in advance ... saying she simply needs stronger boundaries.

Frankie's now the latest member of Ariana's inner circle to publicly have her back ... following supportive messages from mom Joan and word from sources that boyfriend Ricky Alvarez has also been firmly in her corner.