The Very Secret Society of Irregular Witches by Sangu Mandanna

Britt's take

This is the poster child for sitting under a blanket with a mug of tea on a cold day. A lonely witch ends up at a remote house teaching three young witches how to control their magic, and ends up finding the family she didn't know she was looking for. There's a romance woven in, but this is a found family story first. Found family, gentle magic, and zero emotional devastation. Exactly what it sounds like.