Sources close to the singer tell TMZ the pair are getting married Thursday in front of a very-exclusive and small guest list of about 100 people. We're told Britney's brother, Bryan, is expected to be in attendance -- but her mom, dad and sister, Jamie Lynn, will not be there.

Our sources say it's not yet been decided who will give Britney away at the altar, as final details are still being worked out.

BS posted a video of herself with Sam Wednesday and her nails were certainly looking wedding ready as she and Sam rode around town sipping champagne in a Rolls-Royce.

The wedding's been in the works for months ... remember, Sam popped the question back in September, not long after he was spotted shopping for rings.

Britney and Sam have been living it up during their engagement ... jet setting to Cabo, Cancun, Las Vegas and Hawaii ... where they were riding jet skis, booking lavish suites and dining in the trendiest restaurants.

Unclear what Britney will wear for the big event, but she documented a visit from Donatella Versace in March -- so that could've been a big hint on who designed her dress.

Britney made it clear she wanted to get married, buy a house and have kids following the end of her conservatorship ... so she's close to checking off a significant part of that list.

While Britney and Sam announced she was pregnant back in April, she says she tragically suffered a miscarriage ... though BS says the couple is still trying to have a baby.

The couple met way back in 2016 on the set of Britney's music video for "Slumber Party" and have been smitten with one another ever since.

This will be Britney's third time walking down the aisle -- she was previously married to Jason Alexander and Kevin Federline -- and the first for Sam.