The Underwest by Vanessa Grossman

A life of chaos. A fall into a dark world. A way home, but at a price.
Alma has been taking care of everyone and everything her entire life, and it’s left her an anxious, depressed wreck. When her mother announces they’ll be moving in with Grandma, she’s cautiously optimistic. It might bring the stability her autistic brother Waylon needs.
As expected, Grandma’s place is peaceful and calm – but it turns out to be riddled with portals leading into a world of mutants and steam trains!
By accident, Alma and Waylon stumble through, and find themselves stranded in the wilds of the Mutant Federation. There, they meet Sasha, a clockwork android with super strength, but with an unfortunate tendency to break down at the worst possible moments. Sasha promises to protect them until they can reach her guardian who may be able to help them return home.
When Sasha suffers a catastrophic breakdown, the burden falls on Alma’s shoulders once more. It doesn’t help that she has no idea where she’s going, or that Waylon’s condition is deteriorating under the strain.
Can Alma finally wrench control of a life that always seems to sweep her along?