The Solar System is in a delicate balance. The interactions of all the gravities of all the planets in the Solar System have acted in the same way for billions of years.
The only known large-scale event like the one you are imagining that ever is known to have happened is when a Mars-sized planet smashed into the primordial Earth, the result being the Earth-Moon system.
Now there are hints that other cataclysmic celestial events have also happened – mainly the axial tilt of Uranus being almost 90 degrees, which could have been caused by a transit of a large body, and by the slow retrograde motion of Venus, unique in the Solar System, but possibly due to gravitational influence from the Sun.
So, that’s the scientific basis for what might happen.

If a planet sized object were to pass through the inner Solar System, it could impact a planet, causing a massive conflagration, creating a new moon for that planet, and completely rewriting the surface of that planet, at the same time shifting the gravitational balance of the whole Solar System.

It would perhaps only transit the inner planets, in which case it could cause massive tides if it passed close enough to the Earth. Compare the size and distance of the Moon to possible sizes of this planet Nibiru. Gravity follows an inverse square law, remember, so if Nibiru passed twice as far away as the Moon, its gravitational effects would be one quarter of the Moon. This would lead to an extra tidal influence in Earth’s oceans, and when the two tidal influences were added together, you’d get an exceptionally high tide, perhaps several meters higher than a normal high tide.

The closer that Nibiru passed to the Earth, the worse the combined tidal effects would be. If Nibiru passed close enough to the Moon, gravitational forces could rip the whole thing apart, perhaps Nibiru too if it is of a similar size or passes closely enough. The same could be said for Earth.

You would have to also consider how quickly Nibiru is moving. Or rather how slowly. The orbit of planets is quite slow, but a body from interstellar space might be travelling much faster. There might be all manner of interstellar bodies that have been ejected from their own Solar Systems for one reason or another.

And if our Solar System’s delicate balance were to be upset by a new planetary arrival, then one of our planets might be ejected as well. And if that planet was Earth, it would be quite bad, don’t you think?

But no one can know for sure until it is actually witnessed. Our models might be wrong, and we are only human after all.

Fortunately, as you know, this is a hypothetical situation, because, as you know, no planet X or Nibiru has ever been located. It really ought to be visible by now if it was real…

It also has a moon exactly the same size as ours orbiting it, and both are made of Anti-Matter.

If it was roughtly the size of the Earth, it might have to get within several million miles to really do damage, further away than say 20 million miles we might hardly notice it at all.

Venus gets fairly close to the Earth pretty frequently and it does not have a lot of effect on us.

So it all depends what it passes near.

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