He should make whatever investment account for himself that makes sense (traditional or Roth IRA if he's still working and producing income ($5,500 per year allowed), taxable brokerage for the rest of the money - or all into a taxable account if he is not working). He wouldn't be able to name anyone else as co-owner anyway on an IRA because they are INDIVIDUAL retirement accounts... but I'm not really sure that it's possible to name lots of co-owners on regular investment accounts. In any event, it's a very bad idea for the purposes of inheritance to have co-owners.
Once he's established the account(s), then he can name beneficiaries (you and your sibling) in whatever percentage he likes, and in the event that you both have children, there is also an option in the beneficiary designation for "
per stirpes" - meaning in the event that you or your sibling is deceased before him and he forgot to change up his beneficiary designation to include a grandchild... the grandchild(ren) will still receive the same shares their parent would have received.
I have a taxable brokerage account that is actually labeled "TOD" which is "transfer on death" so there is no question that there is a beneficiary named, but naming beneficiaries with the investment company that holds the account streamlines the whole process for the beneficiary. As long as the heirs/beneficiaries are aware of the accounts they are named on, the investment company works with them to make the process pretty easy (at least Fido did, have no ideas about other groups). If you want to make it really easy, your dad can provide all his beneficiaries with the account numbers and the financial group the accounts are with, so you don't have to dig around for that information after his death.
You definitely need to be aware that any beneficiary designation in an investment/retirement account overrules any will designations. So even if your dad stated that he wanted all his estate or a specific account to go somewhere else in his will, if he has the beneficiaries set up in the accounts themselves, they will take precedence over any legalese in the will. He should still go ahead and make a will to cover any and all property, but with the understanding that
any investment accounts with beneficiaries on record will not go through probate or be controlled by the will at all The advantage to this is the accounts pass pretty immediately to the beneficiaries and will not be tied up for months waiting on probate.
All the named beneficiaries for accounts have to do is contact that investment group to inform them that the parent is deceased, provide a death certificate and official letter of instruction, and then either create their own account to transfer their shares to, or instruct them where the money is to go (and can sell off, whatever). When my dad passed away, I had possession of my share of the accounts in under two weeks of informing them (was really only held up by getting the death certificate).
Also, by inheriting instead of being a co-owner, you get everything on a stepped up cost basis. All cap gains/dividends are reset to zero as of the date of death of the owner, and the heirs incur almost no costs selling off anything (depending on how fast the beneficiaries take possession, cap gains/dividend can still be accruing after the date of death). This is a HUGE deal, and a major reason to not put you or any of his children on his accounts as co owners... you'd lose this advantage instantly and it make absolutely no sense to put anyone else on the account as co-owner (this is the same thing for property like a house too... always leave a house to heirs in the will or else you'll be screwing them royally by the stepped up cost loss).
