"Sibling" is Old English, and even less of an Americanism than "mortician" - though unlike that last, it is a very useful word.

In fact the term that tends to be used in the UK these days is neither "undertaker" nor "mortician", it's "funeral director". At least it is self-explanatory and unambiguous. I prefer "undertaker" but I suppose with most people being burnt rather than buried these days, the blackly humorous pun isn't as relevant as it used to be.