Trump will do absolutely anything to win this presidency... Except, clearly elaborate on the details and benefits of his policies. He screams all kinds of pathetic.
To be clear, I don't like Hillary either. I would understand on some, very low, very desperate level how this is supposed to influence Obama. But why should Hillary be impressed?!
No, this will not cost him votes because his voters will cast a vote for him no matter what he says or does. He will, however, gain no new undecided voters. It makes no sense for him to bring Obama's brother because he is running against Clinton, not Barack Obama.
It is a bad stunt, but it is the kind of thing that his fans and many of the people who are undecided will like. It may not be fair or ethical, but it is not going to keep people from voting for him. It has little to do with the real issues though.
It is very unlikely that Trump will win the election. Bringing Malik Obama is just a stunt. People have already decided if they like Trump or not. The people who like him will like him no matter what he does. The people who think he's outrageous will always think he's outrageous. The votes are already decided.