The following excerpt is from The Guardian:
She needs to prove that she isn’t still in the hands of big money. She has only been proving the opposite again and again. I want to vote for Hillary. I just need a reason to do so.
Please explain the meaning of the phrase in bold. I can only guess it may mean that somehow she was involved in some fraudulent financial activities in the past and now she needs to prove that she's no longer, but I'm not sure.
in the hands of: In the possession of; in the custody or under the authority of.
If we consider that collective mentality usually assumes that big money can only be made unorthodoxly then my guess could probably be true. On the other hand, being in the hands of big money might mean that she's controlled by big money, that is, wealthy people, but these are just my thoughts, so please don't get me wrong.