My MC's mom died when she was about 5 or 6. Dad remarried when she was about 11. MC and Stepmom never really warmed to each other (but don't hate each other) and stepmom never formally adopted her. MC and her step-sister, on the other hand, get along great. They are both Freshmen in High School.
Dad's sister is her only known bio-relative. She's been a regular, but not constant, presence in the MC's life, and doesn't have an exactly stable career choice (She's a stunt actress at those Stunt Spectacular shows.) She's moving back to the same city as the MC when the story starts. Step-mom and Aunt do not get along at all.
The story starts with a fatal attack on the MC's father in their house. Aunt has arrived in town and meets the MC at the hospital. The MC wants to go home with the Aunt, while the step-mom claims custody. Dad forgot to cover this in his will.
Who do the cops investigating the attack let take the MC home? How much legal trouble can the Step-Mom cause for the Aunt before the Custody battle is settled, not counting DCFS?
The Custody battle aspect is just something that I realized would be part of the "environment" as I worked out the relationships between the various characters, and isn't a super huge part of the plot, but I like the idea of it being a dark cloud in the background of the rest of the story, and I'd like to be able to use it occasionally to ramp up the drama, but in a believable way. Yes, the step-mom turns out to be the primary antagonist, but the core plot is a "the MC gets in the way of the bad-guy's evil plan" rather than any personal antagonism between the MC and her Stepmother.
Now, I've found a fair amount of research on Step-parents and grandparents gaining custody from an unfit bio-parent or a non-custodial bio-parent after the death of the other, but not much on Step-parent vs. extended bio-family when both bio-parents are deceased (which kinda surprised me.) Step-parent vs. Aunt custody didn't yield any direct information, and step-parent vs. Grandparent got me one forum discussion. From what I was able to glean from the articles I found on 3rd party custody seems to indicate that, without a will, the step-mom and the aunt have about equal claim to the MC before things like stability and other elements to determine the best interests of the child are factored in.