where did you read marriage between half brothers etc - just because there was no written records it doesn't mean that people were not aware of their parents (oral history + recitation of parents was well known in Celtic communities - look at the manx nick names which tend to give parents and grandparents) - incest was jumped upon by the church (see my pages) - marriages between 2nd cousins might be more prevalent in isolated communities and also to keep property within an extended family.

The catch with reliance on the IGI is that you cannot trust any records that are not clearly marked extracted (and even here don't forget these are extracts from the 1911 official copy not the actual register tho that copy is IMO extremely reliable) - patron submitted entries vary from the accurate through to downright invented - you cannot tell - as I've said many times Corlett entries suffer very much from this plague of patron submitted entries. For many families around this period property records are a great help but if I am correct in my supposition that they were a Bishops demense family then there will be no such records as the land never changed hands. Wills, not just only of those of the immediate family, are the other major source of information - if you are lucky then a marriage contract might exist but in the absence of any property to transfer there may not be the need for such.