It's the overlap point that applies here, your family business may be nothing to do with consulting but he's going to be an IT consultant doing IT consulting for the family business and keeping the money. If you are going to sound out a friendly partner, you're going to have to dress up what it is you're exactly doing. I still think there will be trouble should you ever get found out though. On the tax part, no issues unless you don't tell your employer what you're doing, then someone may (or equally may not) spot why your tax code isn't what it should be.