I'm confused by the inheritance rules and shiz. Do they prioritize gender or birth order??...

Yeah exactly. If you are going to make it so that a woman can inherit a title then just be consistent with it! Shit is so confusing

I think if there a male in the family members, then that person will inherit the title no matter whether he is younger or older. So that's why her father inherit the title. Her aunt will be able to get the title if the her father is dead. But, her aunt then get married to other family before Avery's father died so then she is not eligible for the title. But, then the title will be inherit to the next in line only for his offspring when Avery's father died.. But in Avery's case, she was the only child of her father so automatically she inherit the title although she is a woman. But if she decline the title then it will be inherit to her nearest male cousin (which is the child of Avery's aunt.. in other words Mathew the 1st male grandchild of the duke).. But if Mathew decline it then, the title will go to the next male if there's one.. In this case, Benjamin is the 2nd son so supposedly he is the one that will inherent it, but he has done a crime so he also can't get the title.. So then the title supposedly will be inherit to the next child, but like usual male will be will be prioritize 1st. However the next in line is female which is Sophia. So Sophia able to inherit the title.

In other words the title of the duke will first inherit like the below

1. Male is prioritize (no matter if he has an older sister, the 1st son will get the title)
2. If no son were born, the 1st daughter is eligible for it (like Avery's case)
3. If the duke do not has any child or all of his child decline it, only then the title will be inherent to his siblings (male is prioritize). In other words, Avery's uncle Viscount Moldin.

that totally makes sense. Thanks for clearing that