Joint holding of assets as a means of avoiding probate -- how to convince dad it's not a good idea
2 attorney answers
Note that I am not licensed to practice in your state. This is not intended as legal advice. Please check with an attorney in your state.
If all your father has is bank accounts and other money accounts and he is refusing to get a will down or to go see an attorney, the best course of action is NOT to put your name jointly on the account but rather, to place ALL his intended beneficiaries as "beneficiaries" of the account. Every investment and bank account could have beneficiaries placed on it and in doing so, technically turns it into a trust account. What this does is (1) avoid probate and (2) have the money go to all the children equally.
First, laws of Washington would govern all property your father owned in Washington, if he lives here at time of death. If there is real property he owns outside of WA (like in Louisiana), that state's probate laws would govern, but Louisiana laws would not govern here otherwise.
Becoming a joint tenant with right of survivorship (JTWROS) on his bank accounts, which is the default in Washington, would make things difficult, as you suspected. You would become the sole owner of the account assets, once he passed away and any transfers to your siblings would be subject to gift tax. Gift tax comes in at a lower amount than estate tax, so depending on how much you are distributing, it could cost in taxes. If you are worried about one of your siblings challenging, this route opens you up to challenge more than a will would.
I wouldn't suggest putting all siblings on the account. Probate in WA is very affordable and simple and it keeps your father in control in life, then his executor after he passes away. If you are worried about challenges, you want to do everything above the board. But remember, anything can be challenged, even if done right--it will just hold up in court better if it is done right.
You said he has a simple will. Is that correct? One way to clear this up now is to submit this will to the court now: that way it will either be declared valid or not, and be challenged or not, before your father dies.
Some things would depend on the size of inheritance. Your father may be right that he doesn't need a "complicated" will, if the assets are minimal and he wants everything to go to you and your siblings in equal shares.
The last thing I want to mention is there may be other reasons your father wants you on his account. For example, he may need help managing his assets and doing things as he is getting up in age. There are better ways to resolve this issue. One way is that he could execute a durable financial power of attorney, naming you attorney in fact. An attorney could do this for a low fee.
Feel free to email or call if you have questions.