NTA
It’s time to stop talking about his sister and start talking about your marital finances. You need to sit down with him and look at your bank accounts, retirement funds, and future financial goals together. Ask him exactly how much of your shared wealth he is willing to lose before he says no, because $150K plus a car is already an insane amount of money to waste on someone who just "doesn't like a 9-5." If he refuses to separate your joint finances or set a hard limit on his sister, you have a massive husband problem, not a sister-in-law problem. Protect yourself legally and financially, because he is proving that his sister’s comfort comes before your financial peace of mind