At least in New York, Rule 1.7 of the Rules of Professional Conduct suggests that an attorney still cannot take a case even with a written waiver if one client would have a claim or defense against the other party in a litigation. The lawyer has to "reasonably believe that the lawyer will be able to provide competent and diligent representation to each affected client," which I don't think anyone could do when one client is an allegedly domineering parent and the second client is a young adult child of that parent.
I have turned down potential clients for much weaker conflicts. If there's any chance that your honest advice to your client would be to throw their co-defendant under the bus, I don't think you can ethically represent both parties. That's just my personal opinion and where I draw the line but I just don't see how you are fully representing your client's interest when you have this secondary interest to consider too.
https://www.nycourts.gov/LegacyPDFS/rules/jointappellate/NY-Rules-Prof-Conduct-1200.pdf
RULE 1.7.
Conflict of Interest: Current Clients
(a) Except as provided in paragraph (b), a lawyer shall not represent a client if a reasonable lawyer would conclude that either:
(1) the representation will involve the lawyer in representing differing interests; or
(2) there is a significant risk that the lawyer’s professional judgment on behalf of a client will be adversely affected by the lawyer’s own financial, business, property or other personal interests.
(b) Notwithstanding the existence of a concurrent conflict of interest under paragraph (a), a lawyer may represent a client if:
(1) the lawyer reasonably believes that the lawyer will be able to provide competent and diligent representation to each affected client;
(2) the representation is not prohibited by law;
(3) the representation does not involve the assertion of a claim by one client against another client represented by the lawyer in the same litigation or other proceeding before a tribunal; and
(4) each affected client gives informed consent, confirmed in writing.