At todays (22-3-2023) DNA discussion group the chance of your lineage having two people on both the maternal line and paternal line was considered as likely where they had come from a 'tribe', 'clan' or a small community.
Endogamy means marriage between one's own tribe or similar unit. An example is a brother and sister marrying a sister and brother from the same community or a pair of sisters marrying a pair of brothers. This happened in my family and from the discussion its happened in other families as well.
Perhaps it's a story that you might like to share with our members through our Journal. Write to the editor or email her at theeditor@nwkfhs.org.uk