I love it spelt Djuna... I think it's cute and quirky.
It also goes really well wiht your 5 letter sibset
I absolutely love Djuna in that spelling, but I could see it being a bit confusing and it does seem a bit more bohemian than Asher and Hazel. Djuna Barnes is also a very, very strong association- not necessarily bad, but perhaps a bit much to live up to. I think Juna may be the solution- it seems a little more wearable in that spelling and fits in much better with Asher and Hazel. It's a legitimate name on its own, a variant of the ancient Junia. I'd love to see it used in either spelling!
I adore both Djuna and Sibyl, they would make a great twinset! Both also go very well with Asher and Hazel, to me. Hazel, Djuna and Siubyl are all spunky vintage girls! I do think though that, whilst less cool, the Juna spelling would be slightly easier to live with and goes even better with Asher and Hazel.