Financial Responsibility to step children

Any advice on how much financial repsonibility we as a step father have for step children?

Step is 17. Wants a new car. I have twin daughters I have saved for a year for a car. Steps mom says I should make it even for all. I disagree and while I am willing to give some its up to her and her ex for major purchases.

I have no problem with regular maintenance and household expenses. I even give freely now and then for an extra here and there.

I drew the line at major expenses.

Any thoughts?

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