Having mulled it over, I like it. It's nicer than plain
Ethel. There were quite a few of them in the
UK in the late 19th century, with various spellings: -win, -wyne, wynne, wynd etc. -wyn looks more balanced to me, and was most common.
Ethelwyn
Mary, Ethelwyn
Maud, Ethelwyn
Mabel & Ethelwyn
Grace were common combo choices. It seems to work best with short, simple names.