They all have 6 in its outer shell because they are all in the same group in the periodic table. Your VERY VERY welcome (:
A sister chromatid refers to the identical copies formed by the replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad. A full set of sister chromatids is created during the synthesis (S) phase of interphase, when all the chromosomes in a cell are replicated. The two sister chromatids are separated from each other into two different cells during mitosis or during the second division of meiosis. Compare sister chromatids to homologous chromosomes, which are the two different copies of a chromosome that diploid organisms (like humans) inherit, one from each parent.
Homologous chromosomes are chromosomes that contain the same genes but each originate from a different parent. There are two instances in meosis where this occurs, Prophase I: Synapsis of homologous chromosomes allows of cross-over of genes between mother and father chromosomes. This mixing gives rise to genetic variation. Metaphase I-Anaphase I: Disproportiante division of parental chromosomes (aka disjunction) allows for further genetic variation. The separation of the homologous chromosomes is random. That is, a daugther cell can have most of its DNA comprised by mother chromosomes, or mosty by father chromosomes, or somewhere between these two extremes (The other cell will of course have the complemetary set).