Keep that anger OP, it is righteous.
All this middle-class politeness washing around your dh’s family is just a mask. They aren’t pleasant.
There is a difference between a step parent being “good” to a step child, and truly embracing that child as their own. If your dh loved both children as his own there would be no differentiation between them - they get the same financial treatment and the same welcome within the family; he would move heaven and earth to ensure that’s the case.
My mum had a step dad who was “always good to her” but she left school to work in the family shop at age 14 with no qualifications whilst her stepbrother went to university. My mum loved her step brother but they were not close. I do think that the step dad’s unequal treatment negatively impacted their relationship. My mum wasn’t dumb and she could see that she wasn’t the golden child. It was very sad for my mum and also her stepbrother - they both lost out due to the selfishness of the stepdad and his family.
I am so sorry for your situation and the rude awakening and hope you hold on to your anger and find a way to push back on the favouritism of your younger child.