I voted Pearl before seeing all that had been written -- I love the name Pearl but think that you should go with Juliet. I think it might trip up some people, but the syllables and stress are different enough that I don't think it is a problem. Even if you sometimes call her Jewel/Jules I would still do Juliet Ruby, it is just the perfect match with Scarlett Josephine!
I vote Juliet. It ends in "ET," while Scarlett ends slightly differently with the sound "IT." It's not like they outright rhyme. In sound, they may not be an ideal sibset, but in tone I think they're wonderful together. Both kind of alpha-feminine names, fragrant and fancy without being fussy. IMO, it's more important that you love your second daughter's name individually, than for it to make the perfect sister-match. She'll spend much of her life away from her sister at some point, so go with the name you love most.
I think if Juliet is your fav then go with that. If you call your eldest 'red' then you arent going to be saying the two names all that often together and you may come up with a nickname for this baby too?? If last baby then go with your fav name x
Well, if you think about the rhythm of the names, there's a difference there too. JuliET- all three syllables are pretty much given equal weight. They roll into each other, with slight emphasis on ET.. or on JUL if you prefer. SCARlett- two syllables, accent on the first. "LiET" and "LIT." Juliet Ruby- sweet that your daughters would have those two red names tying them together, but it's subtle enough with Ruby in the middle spot.