I am often asked for travel advice since I am a frequent poster here also. I understand your dilema. However, the problem was not really created when your sister opened the account, or when you opened the account. The dilema was created by merging the accounts. I know hindsight is 20/20 but the promotions have rules and they are generally followed. In almost every "new" WP account promotion one of the T&C's is that if the account is merged with an older account you will loose the bonus.

In hindsight, my suggestion would have been to leave both accounts open, only accrue miles in the later account that qualified for the promotions and book your gf's dad on a short quick MR or extra trip to get the third trip in. This would give all the bonus miles in the later account and you could basically just write off the miles in the earlier account.

Unless it was an int'l trip the miles in the former account lost would be more than offset by the 7000+ miles gained in the newer account.

There is nothing wrong with trying to pursue it, but it would only be a goodwill gesture on the part of NW to waive the rules in this case. The agents though should not be speaking to anyone other than the account holder over the telephone or via e-mail correspondence.