adding to @VolvoGirl reply --- (a) your mother ( father's spouse ) needs to file as MFS ( Married Filing Separate ) showing her independent income(s) ; (b) your father needs to file MFS, showing his independent income(s) and declaring that someone can claim him as a dependent ; (c) you file under your usual filing status and claiming your father as dependent -- note however, that by doing this you do not get his personal exemption ( it is ZERO for the years 2018 through 2025 ) but you may impact his standard deduction for being a dependent of another; (d) depending on the state your parents reside in there may be "income split" required between the spouses; (e) even if the Edward Jones shows your mother's SSN as primary , if it is jointly owned then its income(s) may have to be shared , depending on the laws of the state they reside in.
As you can see from the above, before you decide to claim your father as your dependent, you need to be clear about what you are trying to achieve. If all you are trying to get credit for is the financial support you provide them ( and if they live together and depending on the state they live in ) , you may or may not achieve much. Suggest discussing this with a tax professional -- this may cost you a bit but definitely will guide you in achieving the best results under the current laws
Good Luck
