Since she can do whatever she wants with her time and since you're paying for the gasoline I'd recommend to your brother-in-law that he needs to see a therapist about his control issues.
No, actually that would be counter productive and juvenile. You need to have a talk with him to discover the truth behind his problem with the situation (my kneejerk assumption is that your brother-in-law rues the time she spends on you, or feels you're an undue burden on her) and a talk with your sister to determine if you're being an imposition on her, (personally my sisters would use their husbands to deflect a situation).