I think Mansfield Park shows that it wouldn't be too weird for relatives who are brought up together from about the age of 10 to get married. Not just because Fanny and Edmund eventually do get married, but because Sir Thomas and Mrs. Norris have a conversation specifically regarding that concern at the beginning. His concerns at that time were financial and status related, not "ick factor" related, and he seemed to feel that it happened often enough that he should worry about it.
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