Naming siblings with same sounding chinese characters
I never heard of such kind of "taboo". But if you are very sensitive with this for some reasons like you believe in things like 周易取名, you may search 周易取名 on Google or Baidu. I think most of the key/original information is in Chinese.

Anyway, I think 嫣 is a very good name for a girl while 彥 sounds like a boy's name.