A memorial doesn't require his ashes, that's a burial. If you want to have a memorial for your father, there's nothing stopping you from doing that. You don't need to have his ashes in order to do so. Just arrange a memorial, invite the people you want to invite, and fulfill your dads wishes.
As for your siblings, why do you have no contact? Do these siblings have a different mother than you? When was the last time you had contact with these siblings? Start with your family, ask if anyone knows where your siblings are. That's your best bet.
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