The authentic of the will has been debated to death on this board, especially when TINI fans ran things. The will is taken as more or less true because there is another will made in 97 that said the exact same thing as the 2003 will. So, unless someone went out of their way to make two fake wills that says that same thing, except add Blanket, the will is most likely real. If there was another will, there would be witnesses to the signing as well as lawyers and one have come forward.
As for why Katherine was chosen, it is very easy question to answer. Really, who was he to trust with his kids? There were precious few people in Michael's world that he could truly trust. Even some people that fans like and supported betrayed Michael after he died.
Also, you have to remember that Michael practically worship mother. That was not made up for the sake of PR or to make the family look good, he thought she was a saint. If you had such a high opinion of someone, why wouldn't you leave your kids to them. Now you may ask how could Michael think his mother was a saint after she watch him get beaten, defend the abuser, and emotionally blackmail him, and I can't tell you. I would be a lair if I said what was in Michael's heart, but from his words and actions Katherine could do no real wrong in his eyes. Fans thought the same thing, which is why everyone cheered when she gained custody of them.
As for Janet, she's the baby of the family and a girl to boot. That family favors boys, something both Janet and Michael said, so they're not going to take Janet serious no matter how much money or success she has. Among her brother's success she is often side noted. They wouldn't even give her a say in her own brother's funeral despite the fact she was the only one who could actually pay for everything.