Andre,
My understanding (and I am open to correction) - it that in a joint account situation, the remaining balance automatically becomes the 'property' of the surviving account holder and doesn't form part of the deceased's estate.
Now, sometimes people refer to a 'joint account' when they don't actually have a joint account, but what they have is 'signing authority' on the account holder's account - when both parties are alive there is no real practical difference between the two - however when the account holder passes away in this second scenario, the authority to sign on the account dies with the deceased (sorry, as I type this it sounds cold, I don't mean it to be, when I'm concious you've lost your Dad). It may be worth establishing which of the above scenarios was in place.
I would also be interested if you do seek legal advice, to find out if my interpretation is correct.
Hope it all works out.
Regards,
BM