My sister's ex drives a new BMW, a Denali, a Hummer, and a Jeep Wrangler. He also has a speed boat. He has a gorgeous thee bedroom home and his belongings are owned by a business, which has been set up in his Grandmother's name.
The problem is that the fact that he uses these assets does not itself prove that he's the true owner of them or that he's earning more income than he claimed he was earning in the child support proceeding. She'd need to show that the grandmother is not really the owner and isn't deriving the income from the business — that it is really her ex that is the real person who effectively owns the business and gets the income from it. That can be done under the right facts, but she'd have to dig to get them.
All the details matter. For example, how is the business organized—corporation, LLC, or something else? If it is a corporation, has the corporation filed its returns for each year? If it is a S-corporation, have the shareholders paid their tax on their share of the business income? What is the supposed arrangement under which the company makes these various assets available to him to use? Is that arrangement reasonable under the circumstances? When and how did the grandmother end up owning the business? What is her role in the business, if any?
She might consider hiring a private investigator who specializes in financial investigations to see if she can uncover the evidence of what is really going on. Of course, that may not be cheap. She might try hiring an attorney familiar with corporate and business financial matters to participate in drafting the discovery for the next child support modification hearing to help her dig up the kind of facts needed to prove that the grandmother is simply a straw for her ex (not all family law attorneys are knowledgeable enough to do that). Again though, that will cost her some money to pursue.
It's unfortunate, but in this kind if circumstance she'll probably have to expend some money on some kind of financial expert, whether it's a financial investigator, an attorney with a finance background, or whatever, to dig up the facts and unravel what is really going on if she really wants a good shot at a significant increase in support.
Moreover, he has not filed personal taxes in several years.
How does she know he hasn't filed his returns? And does she have enough information to know how much income he had to know he was required to file? Note that simply a failure to file a return by itself is not tax evasion. There is a separate crime of willful failure to file a return, a misdemeanor offense that the IRS does not often pursue, but will under the right facts. If she suspects some kind of tax fraud is taking place, she can report that to the IRS following the instructions here: reporting tax fraud. If the IRS pursues it and collects some tax out of it, she might get a reward of up to 15% of what the IRS collects. But unless she has some reasonably good information on what his income is and how he's committing tax fraud it may not go anywhere. The IRS gets a huge number of tips from ex-spouses, many of them with vague, generalized claims like "I know my ex is cheating on his taxes because he drives expensive cars and lives in a nice house." Few of those, if investigated, would turn out to actually be good cases to pursue, so the IRS only puts resources into investigating those with some reasonably good SPECIFIC information about unreported income, inflated deductions, or other tax fraud activity.
And, of course, even if the IRS does take the case, it doesn't help your sister much since the results of any audit or criminal investigation won't be made available to her. She might get a reward a few years down the line (those most tips don't result in rewards), but that doesn't help her now. If she wants increased support now, a report to the IRS or the state tax agency isn't going to be a huge help to her. She'll need to tackle this herself, and will likely need the help of some sort of hired financial expert to do it.