The only Autumn I know has a sister called Willow and I see it's been suggested several times here. Shame you can't use it, they go together really well.
As they're twins, I definitely wouldn't go with Summer, Winter, Raine, Meadow, etc. Just too matchy, imo. I wouldn't choose another A name either.
How about Autumn and:
Bronte, Cassidy, Darcey, Everley, Fallon, Genevieve, Harper, India, Juniper, Katrina, Leonie, Maya, Nova, Piper, Quinn, Ramona, Saffron, Taryn, Wren, Zeryn