Add the fact that the empire was sent into the future and that shining probabbly joined the royal guard by cadence interfering, he has been confimed to have damselindistress syndrome, formerly known as daphne syndrome before she learned karate or judo or skmething
I think Cadance was foal-sitting Twilight after she become Celestia student, and it was just animators mistake or a plot hole that they did not give Twilight cutiemark.
It would make so much sense that Celestia asked Cadance herself to foalsit her since she would not have much time to take care with her ( ruling kingdom and such ), rather than Cadance showing up from nowhere to foalsit random unicorn filly.
Another theory: Maybe Cadance met Twilight before she got her cutie mark and she became her babysitter. She was aware of the power and potential that Twilight had and alerted Celestia. Maybe Celestia knew about Twilight then and invited her parents to have her try out for the magical school for the gifted or whatever it was. So then, it was Cadence who kick-started Twilight's journey into princesshood