Is the incoming mail using your wife's email account but on you shared address, if so a 'reply' will send it using her details.
First email address is the primary.
Subsequent email addresses run as secondary but unless you separate from your ISP's server OE or Outlook and create separate profile (data storing areas) it will deliver everything to one Inbox. I have spent ages trying to sort this out re profiles, they don't seem too difficult to create but involve actually switching between the two. I solved it using message rules and just one profile delivering to two folders eg Inbox as default and Mail for the second.