Not sure about Keke but Jazmine is very versatile as vocalist. She just doesn't have the stage like Bey to showcase it. Pound for pound, Jazmine is just as good or even better than Beyonce as a vocalist real talk. Beyonce being a soprano and being able to sing higher is her only edge on Jaz. I heard Jazmine scat on the end of a jazz standard like 'Round Midnight that would put Bey in her place quick!
‘Pound for pound, Jazmine is just as good or even better than Beyonce as a vocalist real talk.’ -
Outside of Jazmine having better musicality and wit with her riffs/runs (although she can be a bit obsessive with the downward arpeggio runs) and a solid lower register.. beyonce is still more rounded with her belting/mix voice and her head voice ..not to mention her affinity for dynamics like messi di voce and sforzando that she uses a lot in her ballads and Jazmine doesn’t venture in those arenas
“Beyonce being a soprano and being able to sing higher is her only edge on Jaz.” - Both beyonce and jazmine are mezzos, though.. beyonce just has a higher tessitura, so she has a high extension and has great soprano notes, despite her actual actual voice being lower on the scale
‘I heard Jazmine scat on the end of a jazz standard like 'Round Midnight that would put Bey in her place quick’ - I know EXACTLY what u talkin bout, breh.. she so cold! however her jazz/minor chord stylings is a part of signature and isn’t outside of her soul/jazz/gospel background, whereas Beyonce would go from gospel squalling, to bel canto (operatic) staccatto head note flourishes, to a rap-sung hook dripped in southern drawl with a few jazz licks here and there in her riffs all in a 2 hour set.. u just don’t see stuff like that ..mainstream and underground included
you right. I gotta be unbiased. I just like Jaz more. 

how do you hear those notes in your head and then cohesively put them together with perfect timing and syncopation?! Leaves me baffled every time
Vocal analysts are baffled by that shyt. It's the equivalent of being able to pack insane amounts of notes at the highest vocal registry and it makes her voice sound like bells ringing---like vocal hummingbird wings.