"I can only share that we are unable to move her in on the 30th or 31st because there we have a very important commitment with an inflexible date. We would be able to move her in on the 26th. I do not wish to share the details with you, but it is something planned for more than 2 years, having to do with the death of my little brother and the placement of his ashes. "
I forgot to say--you might not even need to be so circumspect in wording, and could leave out both "I can only share" and "I do not wish to share the details with you", which could instill doubt where there wasn't any before.
Scattering of ashes is pretty personal, and (I hope) understandable, and there's not much else one can say besides polite closings. If they can allow early arrival at all, it seems fairly certain that your attempt will be successful.