Regardless of how much I say this is not a forum for providing specific legal advice, I still get asked for legal advice. Frankly, the following comes close to being a good general question and does raise an interesting issue:
Do you think it is a conflict of interest for a spouse of one the siblings to be the financial planner for the Mother's Investment portfolio and the Mother and sibling’s Investment LLC?Conflicts of interest cover a very broad swath of intellectual territory. The writer did not provide enough facts to really get a good start into that territory. Details are needed - the kind that come from a conversation and not from e-mail.
Generally speaking, a conflict of interest does not mean making a profit off of another. That would put all of capitalism into conflict with one another. When we know that another has an interest that is not completely our own, that is not a fatal conflict of interests. A fatal conflict of interest requires - in my opinion - both ignorance of the conflict and the one party's interest trumping the ignorant party's interest.
Undue influence requires some hold over another that makes the other act in ways that run counter to their interest. Where the rich, old man hooks up with the penniless, young woman who convinces the old man to give her all of her money makes a very good example of undue influence.