The section in Massachusetts law is here; it says "In all actions based on contractual obligations, upon a verdict, finding or order for judgment for pecuniary damages, interest shall be added by the clerk of the court to the amount of damages, at the contract rate, if established, or at the rate of twelve per cent per annum from the date of the breach or demand".
It is not, however, as simple as that makes it sound. Suing a family member is never straightforward, as courts tend to assume that such loans are not purely commercial, and so are not necessarily "based on contractual obligations". Your other suggestion, of a trustee who pays late, is even worse. If you can't find the trustee to ask for your money, there may be no reason for him to pay you; there is certainly no "breach or demand". And 12%, though it may have been reasonable when the law was passed, is considerably above market rate now, so the court might decide that disallowing interest is fairer than enforcing the section (there appears to be no way to reduce the amount added, though amending the law is under consideration).