It's not surprising that you don't know what 'in for yet' means! This means nothing at all. Your problem is that you are dividing the sentence up wrongly.
Look at it like this:
[Morrie was brought in] [for] [yet another series of tests.]
Morrie was brought in = Morrie was called in to a clinic, for example.
What was he brought in for? = Why was he brought in?
He was brought in for [yet more tests] = He was brought in for [even more tests]
He's already had several series of medical tests, and now he's been brought in for even more.
I hope that's clearer.