If he made you a joint owner of the bank account, that might have been a gift at the time he made you joint owner.
The money will pass entirely to you outside the estate, and you have no legal obligation to divide it. If you do divide it, that is a gift from you to your siblings. However, you only need to file a gift tax return if the amount is more than $19,000 per person per year, and even if you do need to file a gift tax return, no gift tax is actually paid unless your lifetime total of gifts and estates is more than about $15 million.
If there's so much money in the account that you are worried about a gift tax return, you might discuss that with an estate planner, or have your father put some money into an investment account instead of holding it in a checking account. An investment account can have designated beneficiaries so it also passes to the beneficiaries directly instead of being part of the estate.
