Britney Spears Praises Sister Jamie Lynn in Birthday Post

Britney Spears has done a shocking about-face ... praising a family member, no less her sister.

Britney marked her 41st birthday with a message to Jamie Lynn ... "It’s my b-day but you’re my heart so I’m thinking about you … congratulations on being so brave, inspiring, and showing guts and glory in your show 🎉 !!! You ain’t alone … if anybody knows what that feels like … I get it 🤧🤧🤧 My baby sister !!! I love you !!!"

This is a 180 from Britney's tirade against her 31-year-old sibling, whom she called "scum" after J.L. released her memoir.

Britney has gone on a tear against her entire family over the last year. She was enraged her mom and Jamie Lynn didn't help her 86 the conservatorship.

Britney had called Jamie Lynn, who's appearing on Fox's "Special Forces," her "selfish little brat" sister, as well as regretting she didn't slap her little sis.

Whatever's changed ... well, maybe it's true -- time heals all.

BTW, it's worth noting -- while Brit seems to be sulking in their rocky past, Jamie Lynn is on to bigger and better things in her own life. After dropping her memoir earlier this year, she's now one of the contestants on FOX's new reality competition show, 'Special Forces: World's Toughest Test.' She's one of 16 celebs pushing themselves to the limit in military exercises.