The number of ways to arrange n distinct objects is simply n! (n factorial), so for 13 unique words (
I don't believe they're allowed to have repeats repeats are allowed, but the probability of getting a repeat for 12 words selected randomly from 1500 is pretty low, about 5%), you could arrange them in 6227020800 different ways, however the final word is a check sum, and I think this will make a very important difference.
Initially 6 billion sounds like a reasonably huge amount, since I thought they would have to scan the entire chain up to the point when the inheritance was deposited, unless they knew the exact block or range of blocks when the inheritance was deposited or created, that would cut down on the time somewhat. So, if they had no idea when the transaction(s) were made, then I think 6 billion would be reasonably safe, though probably not completely infeasible.
But, because there is a checksum, then I think they just have to try all permutations of each set of 12 words until they find one that produces the correct checksum. I believe this means that they would not have to scan the blockchain. The formula for the number of different ways to arrange x objects from a set of y total objects in y!/(y-x)!, in this case 13!/1! = ~6 billion, so assuming there's no repeats, they may have to do this up to 13 times before they find the correct order of twelve words, which would be ~69 billion permutations to check. I'm not sure, but I don't think arranging the subset of 12 words and performing the checksum for the remaining one 69e9 times would necessarily take very long, depending on your hardware, maybe a few hours, or a few days, or weeks, but probably not secure for decades if your meddling grandchildren were determined to get their hands on your precious moneroj before you intended them to receive it.