The problems are:
-Yes Isabel Rochev got control of Queen Consolidated. But I don't know how that effects the Queen's personal family fortune? Unless they were stupid enough to have ALL of their assets tied up in the company. And given that Moira was seemingly pretty sharp, and Robert was no fool when it came to finance, I find that unlikely (especially since Moira also didn't trust Isabel as far as she could through her). And no, there is NO concrete explanation for why the personal Queen fortune disappeared (at least not on the show itself).
-We're given ONE LINE of thruway dialogue about Isabel "making the stock worthless." Now I'm no expect when it comes to business, but I don't think that you can ONLY hurt the stock of a particular investor (since there's no evidence that the company is suffering as a whole). You COULD tank the stock, but it'd tank ALL of the stock. And this would likely result in Isabel getting fired for incompetence by the Board of Directors.
-They didn't spend nearly enough time building up the Oliver/Isabel dynamic to make me buy that he would just hand the company over to her (even temporarily) like that. I COULD have been made to buy it, especially given the Russia episode. But then after that, she completely disappears (except for a couple of cameos) from the show until this episode.
Basically, the writers apparently know nothing about how the financial system works (just like they apparently know nothing about how the legal system works, judging by Moira's trial). It was a poorly thought-out move to "break" Oliver.