If someone dies and halacha says you get to inherit, but civil law says I do, who does? If we both agree to let a bet din decide the matter, then you inherit and the matter is closed. But if I take you to secular court and the court (after all appeals are exhausted) says I inherit, we have a clear question for dina d'malchuta dina. Does the latter ALWAYS apply in financial matters?
Two things I found. First, taking someone to secular court over a personal matter violates halacha: "When a case is between two Jews [ben adam l’chavero], we judge it with Jewish law only." [Chazon Ish C.M. Likkutim 16:1] The Talmud says: "Rabbi Tarfon used to say: In any place where you find non-Jewish law courts, even though their law is the same as the Jewish law, you must not resort to them." [Gittin 88b]
Second, this article I found ("Yerusha and Dina D'malchuta Dina") https://www.koltorah.org/halachah/yerusha-and-dina-dmalchuta-dina-part-three-by-rabbi-howard-jachter-and-martin-m-shenkman-esq seems to answer my question.
-The secular laws of intestacy (dying without a will) generally contradict halacha.
-The deceased violated halacha if he died without a will that follows halacha.
-Dina D'malchuta Dina does not apply to the rules of inheritance. [Rashba 6:254]
-Following the secular court's ruling is theft.
The point of the Rashba, broadly, is that dina d'malchusa dina concerns matters in which the state has an interest; most notably the Gemara talks about taxes and drafts. If two parties can agree to any terms they like on a matter between them, then the state may write a law which applies to the default case -- or a case where the two sides don't/can't agree -- but if both sides are happy with something different, the state is fine with that too.
So if siblings agree how to split family assets, it may be different than what US law says, but that's fine. To call this Dina D'Malchusa Dina -- and this is the Rashba's point -- is to say we can throw away Jewish law in cases between two parties where the state is just as happy for them to apply halacha, mediation, or Ouija board to settle their dispute out of court! If so we'd be doing away with halachic civil law entirely.
In short: this sounds like a case for a beis din.
Just to throw in one caveat -- if I'm selling someone a house, the two of us could stipulate that the terms of the sale follow the law of California; a Malaysian mufti; the defaults of halacha; or whatever else we like. Both halacha and the state are okay with this -- if the two of us agreed to those terms. So absent any agreement, what do we assume are people's terms? In some cases a case can be made that both parties implicitly agreed to the terms of the local state; if so, a beis din could instruct the parties how to handle the property accordingly.