Son-in-law is stock broker/financial advisor and has all of his wife's parents -- 80 years old-- retirement $ in stock market. Stock market dives, parent's portfolio cut in half, one parents dies and spouse is left with depleted portfolio. In both parent's wills, they have designated that their daughter (wife of stock broker) and son will divide remainder of estate once other parent dies.
Since son-in-law has advised parents and his advice results in not only loss to remaining parent, but his advice also influences the beneficiaries when the remaining parent dies, is this a conflict of interest?
A conflict of interest means competing or adversarial interests, a dn you haven't provided any details that would indicate any conflict of interest. Here, since this financial advisor is himself one of the beneficiaries, or married to one, it's hard to see how he has any conflict. It seems he has the same interest as his wife and her parents - to grow the portfolio.
You don't specify any time frames, but EVERYONE has lost momey in the stock market in the last few years.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.