Hi UKmale,
It seems to me that your main problem here is the 100 miles long distance you spoke of. The other guy has the advantage of being easily able to stop by and see her (and her family), whereas for you it is a major undertaking and requires some planning. I personally am not a fan of LDRs, and I would probably back out of this one. However you can stay in a relationship with her, as long as you can accept that the other guy is going to have more contact with her than you can have. Not an automatic deal breaker, but certainly something you'd have to make peace with.
Don't know if that helps, but those are some thoughts.
Sincerely,
Kevin T.