Powers of attorney terminate upon death. It appears that your sister's husband will be the financial advisor after your mother's death, but what is really needed is an executor. Perhaps that your brother i law was named executor.
You are mixing some terminology up, here. There is no conflict of interest because the two jobs are completely separate and take place at different times. Sister acts during mom's lifetime to take care of mom, pay her bills, and handle her affairs. At mom's death, sister is done and the POA terminates. At that point, an estate is opened and brother-in-law manages the estate according to the terms of mom's Will. I do not see a conflict of interest.
You can argue "hypothetically", there is a conflict of interest in every fiduciary situation. The more complicated the fiduciary makes it and the more work that needs to be done, perhaps the fee gets larger, etc. There are rules that severely restrict the ability to "game the system."
Your mom apparently feels that your sister and her husband are best suited to handle these tasks. The fact that she has done her estate planning to begin with is a great thing and should make handling her affairs much easier and smoother.
If you feel that either sister or brother-in-law are not performing their required duties, then you should contact a probate attorney right away, to discuss the situation further.
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