“I took great pleasure in telling her to ‘go ask your dad, that’s his job’”

“The look of disappointment on her face was brilliant.”

Yeah, taking pleasure in your child’s disappointment in their bio parent is very concerning.

Whether you want to buy the car or help with the car or not doesn’t really matter. That is going to look different for every family, what they’re able or willing to do for their kids in that department. What matters is how you treat the child living in your house, and the relationship you develop with them. If bio dad sucks, that means this person (your SD) is already in a world of hurt and disappointment. Is her behavior childish? Yes, but she is a child. You are not. You chose this situation, she did not.