The last Act of the play is comprised of one long scene. Everything which was so confusing throughout the play concludes almost abruptly. The last scene reveals the frail illusions which we bear in the name of love.
Orsino starts threatening Cesario (Viola) of betrayal but the moment Sebastian arrives and the truth that they are actually twins is revealed, Cesario falls for Viola and accepts her as his wife.
It feels like what he said in the very beginning actually fits him. He loves the idea of love, not the person for whom he expresses it. Olivia who earlier said that she won’t love anybody while mourning the death of her father and brother goes with Sebastian instantly.
The long concluding scene disentangles the confusion of identity. The issue of Antonio remains unsolved but he stays on stage and witnesses the whole consequence of actions. It is a conventional happy ending.