I have a joint bank account with my sister that requires both signatures to withdraw money. She recently opened a personal account at the same bank. She transferred all the money into her personal account. Is there anything I can do to get my share of the money back? Can I take legal action against her?
You're going to need to talk to your bank, or at least check the terms and conditions of the account. There are then a number of possibilities:
your sister shouldn't have been allowed to do this transfer. Then the bank is in the wrong, and one would hope they would reverse the transfer without any fuss, though in practice maybe you'd have to make a fuss
or, the account wasn't as restrictive as you thought, in which case your sister was perfectly entitled to transfer her money (in a joint account the money is both people's money) from one account to another
if this is the case and the bank mis-sold the account to you, representing it as restrictive in the way you wanted when in fact it isn't, you may have the possibility of obtaining redress from the bank for this mis-selling. But this seems like a long shot.
In any event, this is all rapidly approaching the point where you might want to consider a lawyer. I'm not one of those.
it is not the bank's fault if the mandate was that either of them can sign and withdraw money. But if it was stated that both were to sign before money can be withdrawn that is where the bank can be held accountable.