How to trace who sent email from yahoo account; family member hacked email account
There are no steps you can take because you don't have access to the server logs at Yahoo!  If this is a legal matter, you might be able to obtain a search warrant for the mail server logs if law enforcement is involved.  This would only prove where the connection was established from, which may or may not be useful.

In the meantime, have the owner of the Yahoo! account change their password to something stronger and ensure they don't leave their iPad or any other device logged in where somebody else can access it.