"Can't believe how beautiful my sister looks in her How Could I Want More Video. OMG!" wrote the pop singer before telling fans to tune into Ryan Seacrest to watch the 22-year-old make her country music debut.
Jamie tweeted back, "Thanks for the support B! I love you."
She also took to Instagram to share her excitement with fans. "Surprise!!!!!! On itunes now!!!! Go get it- RIGHT NOW!!!!! This song means a lot to me... Hope it does to ya'll (sic) as well!! Love y'all!!" wrote the pretty blonde.
In her debut video for the track, which is already a hit on iTunes, Jamie sings into a microphone in a country setting complete with soft lights and a piano.
She sings in dulcet tones with her blonde locks loose and wavy, wearing a t-shirt featuring the words New York above the Big Apple's skyline. A male guitar player strums along in the background.
Jamie recently revealed on Twitter that the tune was touchingly inspired by her 30-year-old fiancé Jamie Watson, who she got engaged to in March after three years together. "My fiance @J_Wiii [Jamie Watson] inspired it!!"
"He has stuck with me thru it all! I mean #howcouldiwantmore???"
Britney fans have a treat in store as the pop princess has also teamed up with her baby sister to record a track for Britney's new album, Britney Jean, which is set for release on 3 December.