I notice that you didn't ask, "How was Michael Shiavo's testimony self-serving?" That shows at least a glimmer of hope for you. You realize that Mr. Schiavo's testimony is a priori self-serving, because as the plaintiff he would be expected to give testimony supportive of his suit.
Your question above, however, is a bit dumb. One would usually be expected to give testimony supportive of his family member. This is well documented even in cases where you might expect otherwise; for example, mothers are notably prone to testify on behalf of their spouses against their own children in child abuse cases. Mothers testify favorably to their children, even when their children are guilty of serious crimes. Most persons are motivated to be supportive of their close relatives and friends.
Thus, when a man testifies that he "din't do nuthin'," this is suspect because he would be expected to testify in his own interest. And when his mother, brother, uncle, cousin and girlfriend all testify, "Tha's ri'! He din't do nuthin'!" their testimony is also suspect, because they would be expected to testify on his behalf.
This is quite well known. It's why, in criminal trials, an alibi from a close relative, friend of significant other is not given much credence without supportive evidence.
We'll get to that. I was most interested in the second half of your, "My point is that there is nothing corroborating self-serving testimony except other self-serving testimony."
But now you've changed it to "their testimony is also suspect". Suspect? What happened to "self-serving"?
Second request. How was Scott Schiavo's testimony self-serving? How was Joan Schiavo's testimony self-serving?