Our unwritten rule was:
If a unmarried brother sits next to a unmarried sister two times in a row (like this sunday, and next one), it means that they are engaged. If for some other reason this should happen, then everybody was sure you were engaged and it could take a long time and a lot of talking to assure everybody you were not.
There were no rules of married persons sitting with someone of the opposite sex, if it were a couple (man and with, next to them a sister could sit next to the brother, no problem).
danny