Parentage is complicated. I explain why you need to set it up.
When splitting a parentage, the reviews will split as well, whatever child had the reviews keeps them. If you are combining children into a new parent, reviews will combine together. I predict sometime in 2019 parentages will no longer share reviews and only independent children will host their own reviews.
Rankings are shared in most categories nowadays between children, via BSR rankings. The best ranking child ranking is shared among the others. We do not feel this impacts other children directly for ranking purposes other than additional sale generation to weaker variations. Simply having a better rank does not appear to impact sales however. When splitting children from a parentage, each child would have its own ranking. As for keyword rankings, these are shared when parented, and split when separated as a child as well.
Based on our experience, it’s best to launch a new product as follows. Attach it to a historically performing well listing as a variation, for a short time period of 2-4 weeks. Even if the other variation has bad reviews. Run a viral launch and generate buzz. Break the parentage right after the marketing drive. And let the new listing stand alone if it’s newer/better. The keyword rankings it gains from both of these actions is substantial. Launching new flavors/variations of well performing products is a good idea. Typically if more than one variation is doing really well I often split them out so they gain more search traction for the brand. But if there’s a star variation, and the rest are weaker, I keep them together. The example above is how I try to make a new star variation on a launch.