My sister and I were both POD on moms checking and savings accounts. Now sister has convinced Mom to make her a joint holder of checking.

This is a very interesting question – I’m not a legal expert of course but most of the Internet materials have to do with joint accounts vs POD accounts. Your question is what happens if an account is both joint AND has a POD designation. Which takes priority at the moment of death of primary owner? The joint ownership – so control flows to other owner – or the POD – it flows to the designated beneficiary on death and bypasses the joint owner?

I found one website with regard to California law which seemed pretty clear. If there’s a right of survivorship written into the joint account, the joint owner takes priority – and the POD only applies when both owners have died.

It states:

“Accounts With a Right of Survivorship

Most joint bank accounts come with what's called the "right of survivorship," meaning that when one co-owner dies, the other will automatically be the sole owner of the account. So, when the first owner dies, the funds in the account belong to the survivor—without probate. If you add a POD designation to this kind of joint account, it will take effect only when the second owner dies. Then, whatever is in the account will go to the POD beneficiary you named.
EXAMPLE: Virginia and Percy keep a joint checking account with several thousand dollars in it. They hold this account as joint tenants with right of survivorship. They decide to name their sons, who are both adults, as POD beneficiaries. After both Virginia and Percy have died, the bank will release whatever is left in the account to the sons, in equal shares.”

https://www.nolo.com/legal-encyclopedia/free-books/avoid-probate-book/chapter1-3.html

I mentioned this so that you can have an educated discussion, not because I have any legal expertise.

I would hope the bank sees that there are PODs on the acct so sis being joint is only good while Mom is living. Then u and sis get what is in the acct as beneficiaries. Call the bank and ask your question.

Speak with an estate attorney or financial advisor.

One of my brothers did that with my daddy. Then I told my brother to back off. I took daddy down, got control of everything - checking, savings, trust. My brothers (all) have money issues. I took daddy to elder attorney got everything in my name and opened trusts for brothers so that when my daddy passed they all had something and I had something. That one of us didn't end up with everything. The only thing I kept was the house with the vow of keeping it so that we all had a place over our heads, or a place to come to if we were in trouble. I am not the oldest but I do have the integrity to keep that vow I made to my daddy.

Your mother likely put your sister on this account so as to make her able to pay bills. Who, first of all, is the POA? Because if mother is competent to make decisions ONE of you (the other serving as second) should be POA and put on the account as such. That person can then write checks using your other's name and with addition of "by _____________ as POA".

I don't know how much is kept in checking, but this is likely the smallest of the accounts. If this one done so Sis, who as you say "is in control" can write checks to pay expenses for her mother/your mother.

If you two don't trust one another I think it's sad that you are co-executors, as it will cause a whole lot of dissention.

For myself, I only had one brother. I trusted him and would have with my life. And when he fell ill he made me POA and Trustee of every cent he had. I was meticulous in its care and followed his instructions, while he was able to make them, to the T.

I would, were I you, relieve my mind of this worry. To tell the truth, were your sis to end up with a bit more money, consider it payment for the work she does in primary management for your mom, in bill paying and etc. Just let it go. Don't let money spell ruin to a loving relationship. Just my own opinion.