Re: what is a sister centromere and how does it differ from a sister chromatid?

The centromere is a DNA sequence on each chromosome on which the
kinetochore assembles.  The kinetochore then attaches the chromosome to the
mitotic spindle apparatus so that the chromosomes can be separated
accurately.  I have never heard the term "sister centromere" (even though I
am a Genetics Prof!), but I suppose you could refer to the centromere
regions of two sister chromatids this way. 


"Chromatids" are replicated chromosomes.  Following chromosomal replication
(this occurs in S phase - all of the chromosomes are duplicated by DNA
replication), the two copies remain attached to each other until anaphase
of mitosis.  These two attached copies are called "sister chromatids".  

If you look at a metaphase spread of chromosomes (in which chromosomes look
like the "X" shape) you will see the two sister chromatids attached to each
other at the centromere (the pinched-in part). 


Any college level general bio or genetics textbook will show you these
concepts and have nice diagrams.  I recommend "Biology" by Brooker or
"Genetics Analysis and Principles" by Brooker.