Out of all the things that she lacks from her sisters, she would be the one with the lack of people's skills. Her older sisters have them. Where Samantha might not be the most popular person at school, Lucy has that one cover. Samantha, however, is the most passionate person Rebecca knows.
So it doesn't surprise Rebecca when she observes David and Samantha at the hospital, and again at the dinner that they share frisson.
After all what do you call two people who are in love?
Her hypothesis is right after all.