You Don't Understand Marriage
Hey Sweetie.
Some of the advice you're getting here may sound a little harsh, but it's true. At 19, you have zero understanding of marriage. First of all, it is WRONG of your dad to show you texts between him and his wife. That could easily be considered a severe betrayal should they split up. I can't possibly understand his motive for doing so, unless you give him a hard time about his new wife and he's trying to develop some kind of secret alliance or camaraderie with you in order to pacify you. Or, maybe, since the divorce, your mom was withholding when it came to his right to parent you, or threatening, and so he's trying to establish some kind of "friendship" of equals with his own kid. Whatever his reason, he's an incredibly insecure and immature man to bring his 19-year-old daughter into his marriage...that dynamic is seriously effed-up and is likely going to end up costing him his marriage in addition to damaging you.
Secondly, most women work now and bring in their own income. It is likely that they bought the car and are making payments together rather than your dad "buying it for her." Plus, with the divorce and custody laws in our country being as punitive as they are, many divorced dads pay exorbitant child support, leaving the second earner in the household to contribute more than her fair share to their joint household bills. If your stepmom contributes to the household then, yes, she is entitled to her share of the growth. Of course, I don't know the particulars of your dad and stepmom's finances...but, then again, neither do you. And they may not be what you think. Legally, a marriage is a partnership of equals, and both members of the partnership own the property together. A marriage is not a dependent relationship, as you seem to think it is. I.E. Your stepmom is your dad's equal, not yours. You say she hasn't contributed to the business...well, neither have you. And I am willing to bet any amount of money that she's contributed more to the business, the household, and your dad's bottom line than you have.
Finally, parents aren't required to leave their children anything, and many don't, choosing instead to leave money to charity. However, most parents do want to leave what they can to their kids, and your dad is probably no exception. I assume that he and your stepmom have wills that leave the partner a share and kids a share, or leave the partner everything with money in trust for the kids after both are gone. Still, it's their business, not yours.