Additionally, might be worth it to review the decree ordering payments - it might well be that the total he’s paying is for child support AND either property settlement or alimony in some form. Courts commonly lump all payments from one former-spouse to the other former-spouse together into one payment order. More efficient. It’s presumed that the parties know how much is being paid and for what purpose without requiring the reminder of seperate checks.
It is unlikely (to put it mildly) that your former brother-in-law will be able to get a reduction in his payments based on how he thinks your sister might be spending the money. If he can demonstrate with clear and convincing evidence that she’s blowing it on heroin or kiddie porn then possibly - but generally speaking, it’s not happening.
One thing to keep in mind is that the charts available online for calculating child support payments are merely guidelines. The court is not bound to use them as anything else (other than as a generally-established minimum payment indicator). The court will assign an amount of child support based on the particular facts of any given case. It could easily be considerably higher than the amount listed on those charts - that would be well within a judge’s discretion. If the amount is substantially higher and your former brother-in-law is being driven into bankruptcy by the payments and only the payments then, possibly, you might get a ruling by a different judge that the first judge was unreasonable. I’m not going to recommend holding your breath though.
It might be advantageous to point out to your nephew that the amount paid by his father for his support is under no circumstances his own personal money. There is a reason courts do not order those payments paid directly to the children. I wouldn’t be surprised in the least to discover that your former brother-in-law had nothing to do with his recent questions. He might be thinking that money is his and have grand plans for what he’d be spending “his” money on if only his mom weren’t selfishly keeping it all from him.
Keep in mind, I’m not your lawyer. My comments are for academic purposes only and may not be reproduced without the express written authority of Major League Baseball.