Briony and
Jenna or
Briony and
Keely.
Jenna sounds a bit better with the brothers' names, I think.
Avery and Ellison are all masculine to me.
Briony and Brinley/Brenna/Breann are a bit too matchy, though
Brenna might work.
Paisley and Kea don't fit with the siblings' names at all.