I think they will pull some of this cards
1. Flash sentry will be wondering around new twilight, she will not be the same as the other, but will end up being just a friend, and from there will born a justified love betwen the new couple.
2. They say something like "just like friendship is eternal multidimensionaly, love is to, so that mean they will fall in love just because one is twilight reflection"
3. They just fall inlove because flash keep being nice to new twilight, and she fall into his enchantment and coolness.
I dont know, he is just such a lame tool that i dont know what they want to do with him.