@deogratias4
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As to your immediate problem, if it were mine, I’d have two goals. The first would be not to offend Our Lord, and the second would be not to offend my “new” oldest brother.
Initially, I’d go to a jeweler and ask that he put the ashes into a separate container, replacing them with a similar substance to make it appear as if your father’s ashes remain.
Over and over, again, you’ll hear on CAF, “Ask your priest.” If he doesn’t know what you should do with the ashes, since such a tiny amount is involved, and since you have no way of including them with the rest of your father’s ashes, I’m confident that he’ll give you directions after consulting with his bishop, if he doesn’t have an immediate answer. I’d follow his directions for the separate container of ashes. You could use a clean pill bottle, or buy a new, unused one from a pharmacist.
You’ve not mentioned your thoughts regarding your “new” brothers. I suggest kindness and understanding, and leaving the door open to future communication. Your—and their—father’s sins of commission and omission are only his own, not theirs, nor yours. If your morher is living, perhaps she’ll fill in the gaps.
You five children, though, now have the capacity to learn to accept each other and enrich your lives. Unless they are toxic individuals, interacting with them would seem to be a series of positive events. For one thing, brothers are usually great for running interference for their sisters. Presenting you with the vial of ashes on a necklace took thoughtful planning that indicates a desire to include you. Were I in your shoes, I’d do with the real ashes whatever the priest tells me, then make every effort to keep my brother(s) from ever knowing, as the oldest brother—and perhaps all of them—thought they were doing something that would make me feel included, like one of them. I wouldn’t want them to ever think otherwise.
However you resolve your problem, I’ll put you and them in my daily prayers and hope that the necklace cements your relationship with them.