When the US legalized gay marriage (at least for now), one of the talking points against it was that men could marry their sons to avoid the inheritance tax, the same idea you are talking about. But the fact is fathers could marry their daughters and mothers could marry their sons to avoid taxes before that law was passed, yet it wasn't done. It was all just a smokescreen.

The other funny thing was that the people against gay marriage by and large were also against inheritance taxes, so you'd think they'd applaud a new loophole.