This is a pretty common trope, although not necessarily with an age gap--someone starts a relationship with a relative of an ex, or even someone who just looks like the--usually tragically lost--ex.
The traditional resolution is, "I don't love you because you're like her--I love you because you're you."
As I've said, I don't much care how this story resolves as long as it stays as thoughtful and as well-written as it's been. But there's no utility in predictive lamenting.