Translation: Chinese relative names + numerals into English
What about this construction:

Jack the Ripper

So we can have something like

Uncle Wang the Second

Etc.

PS: yes, Mrs. Li the Second cannot precisely convey the meaning of 李二嫂; even Uncle Wang the Second is a little ambiguous. I doubt whether there is actually a concise expression in English, so maybe I will leave a footnote to explain it in the article/book/etc.
PPS: According to the wikipedia entry Chinese kinship, there were translations like

Second Uncle
—Yang, Rae (1998), Spider Eaters: A Memoir, University of California Press, p. 46, ISBN 0-520-21598-2, retrieved 2009-07-14, &quot;The letter became evidence of Second Uncle's yearning for the Nationalists to come back.&quot;

Thus, Second Uncle Wang sounds quite reasonable too.