Believe me, I know exactly how you feel. Ever since I stuck my neck out and said "Planet X" is fake, and "Nibiru" is fake, I've had people trying to behead me.
Here's the facts:
If a planet the size that is claimed existed anywhere close enough to cause the kinds of effects claimed, we would be able to see it.
Lieder's claims are so muddled that it is difficult to make out what she means. In some places she describes planet X as "4 times the size of earth" and "23 times the mass of earth" and "highly magnetic" and "smoldering". In other places she claims that it is a 'brown dwarf' or 'red dwarf' star, and heavier than Jupiter (Jupiter is about 318 times as massive as the Earth). When her errors are pointed out to her she tends to react badly, and blame our "limited Earth science", or accuse the person of being engaged in "disinfo" (disinformation).
Sitchin's Nibiru is based on his own unique interpretation of a cylinder seal, which he claims shows 12 planets. When his interpretations are shown to be incorrect, he reacts by claiming that the scientists (who are using literally hundreds of years worth of translations) have it all wrong, and only he can interpret the seal correctly.
There is no planet X and there is no Nibiru. The described objects are impossible, on impossible orbits, and posses impossible attributes. Any planet on such a long and narrow orbit would either drastically affect the orbits of the other planets and/or be ejected by the gravitational effects of the gas giants.
They are totally bogus. Anyone who claims that they know where these objects are can collect an easy $5,000 by providing the coordinates at http://nibiruchallenge.blogspot.com/ (not our site, by the way)
In the past people have provided coordinates that turned out to be one of the known planets, the great nebula in Orion, the Andromeda galaxy, the Pleiades, or other known objects.
Any object of the size claimed on the orbit claimed would be visible and would be moving rapidly, and would have attracted the attention of the hundreds of thousands of amateur astronomers worldwide. Claims that it can only be seen from the South Pole are bogus too, because it's been a few years since it was supposed to be here, and therefore the entire sky over the southern hemisphere has been viewable (unless of course it's invisible, or performing an impossible inward spiral, or (most likely) doesn't exist).
So ask your brother and your friends: where is it? Have them provide the coordinates. I have an account with a global telescope rental company, and I can image any part of the sky within a few hours.
The proponents of these objects have never been able to meet this challenge.