That's not what I was looking for. The presumption, I believe, is that one's legal guardian is acting in one's best interests. Under what circumstances is that presumption overturned, and outside representation required - aside from the obvious conflict of interest, that is.
"Conflict of interest" is the best I can do, sorry. There's always a little conflict of interest in any relationship, so there's sliding scales and balancing tests.
For lawyers, at least, there are mandatory ethics classes, and mandatory annual refreshers, and monthly mailings from the State Board setting forth ethical rulings and disciplinary proceedings, and if you're stumped you can call the State Board and get an advisory opinion.
But none of these work if the lawyer in question is hubristic, loony, or delusional.