No, I don't think his siblings could have paid, using any monies from their parent's estate. Any money belonging to the murder victims is off limits to be used for Nick's defense. IIUC.
The California Slayer Statute (Probate Code § 250) prevents anyone who intentionally and feloniously kills a decedent from inheriting from their estate, disinheriting them from wills, trusts, insurance, or intestate succession, essentially treating them as if they predeceased the victim.
Disinheritance Scope:
The killer loses all property, interests, or benefits from the victim's estate, including will bequests, trust distributions, intestate succession rights, life insurance, and fiduciary roles (like executor)
Application:
The killer is barred from receiving any inheritance or benefit from the victim's estate
Distribution:
The inheritance passes as if the killer predeceased the victim, avoiding loopholes
So the way I understand it, all 3 siblings are getting their money from the parent's family trust. This law says NONE of that family trust can go to pay for the killer's defense. It doesn't matter if one of the siblings tries to pay it in their name or not----the money cannot come from the victim's estate.
If the siblings have another source of money that can pay the bill, that's fine. Maybe an aunt or uncle have their own income stream. But the financial estate from Rob and Michelle are legally off limits to pay for the defense team, IIUC.