Hi - your mum's version follows the primary rule of translating poetry - translate the meaning, not the words. I'd say it's a very good translation. I prefer not to even attempt to make (or sing) a singable translation, so I've always just given literal translations. I don't think I'll ever find an English version I like as much as the original German.
And I don't think I'll ever find a German version of "Edelweiss" that I really like.

-Joe-