Couple of things could be happening here.
One, if you ever typed the address in, and typed it incorrectly once, then the auto-complete would select that misspelling first. You would have to then go in manually and change it. I would assume that you have it spelled correctly in your Address Book, so when you enter it from there directly this isn't an issue. However, if you are typing the address in manually, the auto-complete would attempt to fill it in using the misspelling first.
Hitting the "reply to" button should avoid this, unless the next scenario is what is happening.
The second cause could be that your cousin may have set up their "reply to" address wrong on their end. That would account for your email going to the wrong address when you simply hit the "reply" button in Mail. E-mail providers almost always allow you to set up a "reply to" address, so if you prefered a specific email address, then no matter which one you send it from, it will always be replied to that address you specified. This is possibly where your cousin has made an error. This is something that you cannot change, because your cousin would have been the one who set it up.
For example, I have about 9 different email accounts. For most of them, I have my GMail account set as the reply to. This way, the email always comes back to my GMail, no matter where I send it from (providing the recipient uses the "reply to" button).
