I'm glad these two were brought in. It shows that even before she was willing to accept it, Twilight always knew deep down of the true value of friendship and love. Shining and Cadence are her oldest friends. They're essentially the reason she knows what friendship is to begin with. Twilight's status as the Element of Magic and Princess of Friendship is a beautiful flower. As such, her relationship with her brother and her (now) sister-in-law is the seed of that flower.