I would be mad too.
"Perhaps it is time to scale back on the assistance you provide to them."
Kiki (hit my magic age of 45 and no longer TTC),but mom to a beautiful teen DD & 2 angels in heaven & married to my best friend
You're right that its not fair and it sounds like she has been taking advantage of your generosity. Sometimes people are so self-absorbed that they don't realize that they are supposed to return favors. Maybe its time to have an honest talk with her and tell her straight up that you're surprised that she expects payment for this favor because you have done XYZ to help her out. (does she pay you for gas when you drive around her daughter, do their shopping, etc?) That gives her a chance to recognize and appreciate your efforts and remember that she needs to reciprocate. By her response you will know if you ought to reconsider how much help you will give her in the future.
I agree with elc11 that it might be a good idea to sit down and have an honest talk with your cousin.