In the past, the 12th Planet's orbit was farther out in your Solar System, but over the eons your Sun has lost mass, and the orbit now comes between the Earth and the Sun. What is the risk of collision between the comet and the other planets in the Solar System? Where the Earth and the comet were in each other's path in the past, there was more than the element of chance involved. You, on your highways, have hurtling missiles on a collision course with each other by the millions, yet you have very few collisions. There is an additional factor in place in planetary collisions, as they are no more without drivers than your hurtling automobiles. All planetary bodies have attraction/repulsion factors at play, and when they come close to each other they in fact push each other away. This in almost all cases suffices to prevent collision, although for any inhabitants it is a shaky experience.
Where actual collisions occur there is a difference in body size, with the smaller object traveling at great speed. The speed overcomes the repulsion, and the smaller object also becomes caught in the gravitational pull of the larger object. Such is the case, for instance, when meteors fall to Earth.
There are remnants of planets between tiny Mars and the giant Jupiter, which once held the potential for life such as your Earth now holds. These water planets met their death during the Asteroid Belt confrontations, where missiles went every which way during the passing comet's journey. Should your Solar System not have been so disturbed, you would not be counting the comet as the 12th Planet (or actual 10th orbiting Planet) but as the 24th Planet. Most of the planets that were destroyed were tiny, like Mars or less, and quite vulnerable to destruction by a larger traveling body. They became caught in the gravitational web of the traveling monster, drawn in to become moons, or what we call travelers, and it was one such that struck the Earth early in her life, and gave her the wound that is now the deep Pacific Ocean.