Winterfrost by Michelle Houts is a lovely little GU story, although there isn’t a very formidable adversary. Still, it has the classic “search for baby sibling” feature, which is of course a big part of the original inspiration for Girls Underground – Labyrinth – and even one of its original inspirations, Outside Over There.
Bettina, 12, lives in Denmark with her parents and baby sister Pia. Unbeknownst to them, their barn is occupied by a nisse, a magical tiny creature who helps care for the family and animals. When the family gets bad news during Christmas, they neglect to leave the customary gift for their nisse, who gets upset at the slight. Bettina’s parents leave for a week, and she is left to care for Pia alone – but the nisse takes the baby to the forest. Bettina goes searching for her, only to find that Pia was stolen again, by another nisse who may be more dangerous (he qualifies as the adversary, but only just). Bettina meets the other nisse of the forest, and goes underground into their home (becoming temporarily tiny herself), enlisting several of them as companions to help her.
She travels to confront the nisse who stole her sister, growing and shrinking several times along the way, just like Alice. At first the adversary gives her a task to complete, but eventually she wins her sister back, and even helps the unhappy nisse reconcile with his family (one of the few times I’ve seen the girl help the adversary rather than defeat him).