It was for the judge to determine the credibility of the witnesses. And since Michael's brother and s-i-l have no dog in this fight, they look to be "independent" witnesses (which is not to say they are not biased...again, for the court to determine).
I agree...however, taken in toto, there is/could be an underlying motive for the witenesses the shade the truth (I'm not talking conspiracy) They probably know that Michael is seeing/involved with another woman. They feel that it is better to let Terri go and for Michael to move on with his life. If the question was asked in a broad general way...."Have you ever heard Terri express an opinion about being on life support..." Once the lie is on the record..it's tough to go back and explain yourself. Now with all the media scrutiny, how do you put the genie back in the bottle.