Britney Spears Got Heads Up on Jamie Lynn Joining 'DWTS,' Well-Received

Britney Spears wasn't blindsided by her little sister, Jamie Lynn Spears, joining "Dancing With the Stars" -- in fact, she was fully aware and approved ... TMZ has learned.

Sources with direct knowledge tell us, Britney got a heads up about JL appearing on the competition show before the news came out -- and we're told Britney actually took it well.

No word on who exactly delivered the update to BS, but our sources say she was fully supportive of Jamie Lynn ... and, apparently, even excited for her sibling.

Now, again, we don't know who looped Britney in -- but whoever did clearly did so with their famous rift in mind, as they've had a rocky history. At times, Britney embraced her sister ... but, she's also trashed her. A lot stems from JLS's book last year.

The status of their relationship has felt a little iffy at times -- especially as Brit has continued airing grievances about her family at large on social media -- but after hearing this ... it would appear the Spears sisters are on somewhat good terms right now.

Jamie Lynn is getting seriously accommodated ahead of her time on 'DWTS,' which kicks off Oct. 10. She's been allowed to rehearse near her home in Tennessee, on account of the fact she's a hands-on mom of 2.