I agree with the statements made by the other respondents, but make one comment which you should be prepared for. Setting up a limited company is not an indication of wealth in itself. If he has a certain standard of living, it is quite possible he is living off the loans he took out to start up the business, until the business starts to make money. So his income may be that low and "the company" is lending him money. Stupid, I know, but an awful lot of people setting up businesses do it! You really need to find out how the business is actually doing and if it is a new startup, you may have a long wait before he has to publish accounts, and even then, they may not tell you a lot.
You need to find out wat kind of business it is and is it a cash payment business. What kind of overheads has it got, vehicles, offices, capital equipment, office equipment. stationery etc etc? I'll bet most of it is on borrowed money, otherwise wouldn't your sister have known if he had a ton of cash out by to lay out on all that stuff?
I repeat I am not agreeing with what he is doing, but this may well be the defence he will put up when challenged and your sister should be prepared for that.