In case a retail trader dies, how can his or her relations gain access to the money in his or her forex account?
If it is a joint account, the surviving joint account holder named has rights over the funds in the account.
If any other person wishes to contest this they need to go through the probate legal processes applicable to any assets. They should be able to apply for a court order freezing the assets in the joint account while this is processed.
Similar if its a single person named account - the person who wishes to claim the funds must apply for probate, but the account is already frozen on the death of the client.
Sometimes surviving beneficiaries do not know where accounts are, so this can mean money left lying dormant in accounts that only the deceased was aware of. Could be a big financial issue for surviving relatives.
A relative or lawyer could be given enduring power of attorney before death by the client - this would mean they would automatically have power to freeze the account and possibly even to withdraw money from it without going through the courts.
All depends on the laws where you live of course.