Here's my opinion--for what its worth.
You pay CS to your sis for your son, right?
She pays for him to have a roof over his head, gas and electric to keep him warm and out of the dark, food for his body, clothes for his back--these are just the basics.
Then, she pays for gas to get him back and forth to school, boy scouts, church, soccer...whatever.
She also pays his school workbook fees (They are $60 per child where mine go to school), school lunches, field trip fees...whatever.
Then, what about the extra stuff like rollerskating, movies, going out to eat, laser tag, computer resources...whatever.
MY POINT IS:
What alot of parents who pay CS fail to realize is that yes, your sis gets the check. She may not use that money to open a bank account for your son, but she is paying for his upkeep and his care while addressing his needs. She may take that CS check and make a car payment with it, but she is still taking money from HER OWN account to care for YOUR child. The money you pay would serve to pay her back for the money she supplied to care for your child. --AND BELIEVE ME...THE CHILD SUPPORT YOU PAY IS A MERE FRACTION OF WHAT IT COSTS TO RAISE A CHILD.
You can't think of it as "YOUR CHILD'S MONEY" because it is not. It is for his upbringing and general care, which she is providing.
If you want your child to have money, open a bank account for him yourself.
It costs ALOT of money to raise children. Collectively, I have 6. (3 of my own, 2 I have custody of due to abuse and neglect, and one that belongs to my b/f).
My best advice to you would be to say, "GET OVER IT".
Don't mean to sound like a jerk, but if you let that hate fester inside of you, it will undoubtedly affect your child in a negative way at one time or another. YOUR CHILD IS WHAT MATTERS HERE, NOT YOU OR HER.
Remember--she's taking money out of HER pocket for YOUR child. Plus, she's spending the time with him that he needs. Lighten up on her. Sounds to me like she's doing you a big favor by providing your child with a home in the first place.
Oh---and DON'T EVER speak badly of her to your child, or let your child hear you say anything negative about her. To me, that's child abuse.
Just my opinion. I am sorry if I offended you, but that advice is from the heart and from experience.
Sky