No, but I would be surprised if she would accept the work if asked. She has a direct conflict of interest in that she is potentially financially involved with your decisions. This makes it a practical impossibility for her to be a fiduciary.
Don’t know your qualifications to give this sort of advice, but I’ve been practicing law for 38 years and I think you’re wrong.
Under settled rules, the lawyer in this case has a conflict of interest only if the OP wants her to take a position adverse to another client (or if another client wants to take a position adverse to the OP). And even then the conflict of interest can be waived in writing (by both clients). Some law firms, mine included, obtain conflict of interest waivers from most clients in advance, so as to side-step these issues.
There are often sound reasons not to engage relatives. I’ve never been on either end of such an engagement, and probably that won’t change. My relatives can’t afford my fees, to begin with. And I need to be able to fire my clients in case they prove to be difficult — it’s hard to fire a relative. But there would be no disabling conflict of interest precluding such an engagement except in the unusual circumstances identified by me above.
Likewise, any partner or associate of mine would be equally free to accept the engagement.
All of this assumes the lawyer has no role in the transaction other than a lawyer’s role; i.e., fee for service, or no-fee for service. There are ethical rules, not discussed here, that regulate lawyers’ business transactions with clients. But the mere status of being married to a potential heir of a client involved in a transaction is not a conflict of interest under prevailing rules. Indeed, some lawyers are hired by law firms precisely because of their family connections.
The best reason not to use a relative for estate planning is that other relatives will suspect, and might assert, inappropriate influence if they are unhappy with the results. The estrangement can be perpetual.