Do the spouses incur sin if the wife thought she had purified herself, so she prayed and her husband had intercourse with her, then she thought she saw blood so she did not pray Fajr, then she bathed and prayed Dhuhr, and is expiation obligatory in this case?

If a woman has predominantly become pure and the signs of purity have appeared, so she purified herself, prayed, and her husband had intercourse with her, then there is no sin upon them for that; because what is forbidden is intercourse with her while she is menstruating. If the menstrual blood returns to her, she becomes a menstruating woman, and prayer becomes forbidden for her, and it becomes unlawful for her husband to have intercourse with her. So, whenever menstrual blood descends, its ruling is established, and the woman should not hasten ghusl and prayer until she sees the white discharge or complete dryness.