Well, Big Mac is not only a tank, but also strong damager too, he is just slow and not to accurate, but I am sure that if Twilight will use telekinesis to keep enemies immobile, he will have no trouble to hit them with full power.

As for Twilight, she is not just strong damage, I bet she can learn healing spells to keep Big Mac alive, buffing spells to make Big Mac even stronger, support spells to make enemies slower and weaker, and we can not forget about barrier, Twilight may be far weaker in barriers then her brother, but if Big Mac would not be able to tank enemy attack, Twilight can always put on a barrier to let him take a break.

So yea, Big Mac is strong Tank, Twilight is support/damager, they are both very flexible and can adapt to battlefield.