I adore Sibyl (this is my favourite spelling too) and it sounds great with your set.
Sibyl Eliza - I did like Elizabeth but Sibyl Elizabeth "reads long" compared to Sibyl Eliza, IMO.
I also really like names with the accent on the 2nd syllable, like Marie (but Marie is used-and-abused as a middle so I'm trying to think of others)
Sibyl Irene (I like the image of an oracle of peace)
stripedsocks- Thank you for the thoughtful response! I too think Sibyl Eliza might be a better fit with my kids names. I agree with the accent on second syllable names and think they pair well with Sibyl. Rose, my daughters middle name is named after her great grandma, whose sister is Irene... so Sibyl Irene has a family connection too. I also like Irena. Do you think Sibyl Irena works? Funnily enough her initials would imitate her name - S.I.B. if I chose I for a middle name. I also am drawn to Sibyl Elise but like the 3 syllable middle name I think (though I am not going to chose a name based on this)
Probably too late now :) but mine is Therese.