If the accounts were in joint tenancy, which is quite common, then the surviving joint owner automatically owns the balance upon the death of the other.
This is one of the biggest mistakes people make in estate planning. Oftentimes, children end up receiving money that the parents didn't really intend for them to receive.
I often end up counseling children that, although they legally own the bank account, they might want to consider dividing it according to the deceased parent's will.
Now, if the account isn't truly "joint," and they were just co-owners, then the co-owner child is only entitled to the portion that she personally contributed. However, since she was able to take it out, I'm assuming that the bank verified that it was joint with right of survivorship.
If this doesn't help, then I'd suggest checking with an attorney just to be sure.