I can't send emails to my wife's account but she can send to me. Today when I tried the email came into my own in box, addressed to my wife's email!

Maybe she has configured settings in her Yahoo account to redirect your email somewhere else, for example, to consolidate email accounts. Or maybe she accidentally tagged you as spammer and blocked you. Can she received emails you send to her from the Yahoomail website (not from the iPad)?

Check that the proper SMTP server is indicated in the Yahoo account settings. Also check the ports being used for the outgoing SMTP. Google for the proper settings.

https://help.yahoo.com/kb/mobile-mail/server-unavailable-error-ios-mail-sln8834. html?impressions=true

Yahoo has messed with their stuff lately (honestly they've been messing with their servers for months)

One thing would be if you guys wanted to use the yahoo mail app, or there have been other users posting solutions that they say work for them

https://discussions.apple.com/thread/6888118?answerId=27906475022#27906475022&ac_cid=tw123456#27906475

https://discussions.apple.com/thread/6804040?answerId=27894621022#27894621022&ac_cid=tw123456#27894621

These have applied to people not able to get mail but changing the settings may help you and your wife. (My yahoo is still working so I haven't needed to try any of these)

I've noticed with my Yahoo that they've changed their login protocols. For example 'stay signed in' is always clicked and the push is for people to merge multiple accounts to access their multiple yahoo from one account/sign in instead of signing out/in to access different accounts. I'm wondering if that change is what's messing with the protocols needed for iOS mail to work. But no one really knows for sure (and neither Apple or Yahoo will comment on the issue)

About all users can do is find work arounds that work for them.

If these don't work or if you don't want to use the yahoo mail app, you might try using safari to go to yahoo.com and access your mail that way.