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« on: Saturday 23 May 20 02:04 BST (UK) »
If I understand right, you are asking why DM seems to link with descendants from your grandfather through both wives (Emma and Mary Ann), but does not match with other members of Emma's family?
The chance inheritance of DNA can explain the latter point. 2nd or 3rd cousins do not have to have DNA matches with each other since in theory they only inherit about 6-12% of their DNA from their common ancestor. You have inherited 'different bits' of DNA that match two other sets of 'different bits'. of DNA, but those two other sets of 'different bits' don't match each other. That is not unusual. And if John, Caroline and Emma were not full siblings (i.e. one or both of their parents had them by different spouses), that would make the chances of common DNA even less.
Regarding the apparent connections of DM to your grandfather's two wives - have you considered that your grandfather was related to one of Emma's parents? Or maybe John's wife was related to your grandfather or to Mary Ann? Or that the grand daughter of Mary Ann was married to, or had a parent who was related to, someone from Emma's family, or even to John's wife's family? Or maybe one of DM's parents was related to your grandfather, or to Mary Ann.
There is no simple answer - only more research.